PARIS (AP) _ Visa requirements imposed last fall to combat terrorism are considered successful, even though many visitors' passports are not checked, a government official said Monday.

A spokesman for the French Foreign Ministry, speaking on condition of anonymity in keeping with government policy, said the passports of people entering the country by car are not checked as regularly as those entering by plane, bus or train.

Spot checks are done on cars entering France from any of the eight adjacent countries. Paris car rental agencies say only one out of every 100 cars is stopped by frontier police.

Nevertheless, France is satisfied the program has been effective in dissuading terrorism, the official said in a telephone interview.

''Terrorists know we're checking carefully, even if we don't stop everyone, so for them there is the threat that if they try to enter with explosives, for example, they could get caught,'' he said.

The visa requirements were imposed Sept. 15 in the midst of a series of terrorist bombings that killed 11 people and injured more than 150. Prime Minister Jacques Chirac announced that visas would be required for citizens of the United States and 40 other countries who previously did not need them.

The requirement initially was imposed for six months and has been extended indefinitely.

A visa is a stamp on a passport showing it has been examined by officials of the country being visited and granting the traveler passage into the country.

The visa requirement initially triggered some protests from travelers, inconvenienced by long lines at French consulates where staffs were ill- equipped to handle the flood of requests.

Some Europeans were indignant.

''A passport should be sufficient,'' Swedish Foreign Minister Sten Andersson said last November after refusing to travel to Strasbourg for a session of the European Parliament. ''I am irritated over the French visa requirement for Swedish citizens.''

Describing his canceled trip as a protest against the requirement, Andersson added, ''This has no effect on terrorism. This is only an act to calm public opinion at home.''

The French official said the visa requirement complements other measures such as increased police patrols, greater cooperation between international intelligence services and a French judicial system that ''is very severe on those charged with terrorist activities.''

He said U.S. tourists have been cooperative despite the inconvenience.

Marvin Groeneweg, head of the visa section of the U.S. Embassy in Paris, said he has received at least two calls a day since September from disgruntled Americans turned back at French airports because they did not have visas.

These travelers usually are sent to the French consulates in Brussels or London to obtain the visa stamp, he said, adding that the U.S. Embassy can't do anything to help travelers without visas.

Americans planning a French vacation this summer should apply for visas in person or through the mail at least one month in advance, Groeneweg said.

Carole Harris of Minneapolis applied through the mail for her visa from the French consulate in Chicago and said she received it within two weeks.

Mrs. Harris, interviewed at the Tourist Bureau in Paris, said she and her husband arrived by train from Germany and had their passports carefully checked by frontier police.

''They got on at the last stop in Germany and got off at the first stop in France,'' she said. ''I don't know what they would have done if we hadn't had a visa. It was worth the effort, after all.''