BRUSSELS, Belgium (AP) _ Common Market countries, reporting that the number of AIDS cases in 13 Western European countries has nearly tripled in one year, announced a grant of $492,000 for AIDS research.

The announcement was made in Brussels on Thursday in conjunction with the openign of an international conference on AIDS, or Acquired Immune Deficiency Syndrome.

The number of reported AIDS cases had jumped from 421 in July 1984 to 1,191 in June 1985 in the 10 European Community nations, and in Spain, Sweden and Switzerland, the countries said in a report.

At the end of that period, there were 392 reported cases of AIDS in France, 220 in West Germany and 176 in Great Britain, the report said.

The 10 nations of the European Community are Britain, France, West Germany, Italy, Ireland, Belgium, the Netherlands, Denmark, Greece and Luxembourg.