TOKYO (AP) _ Ethiopia's ambassador to Japan left for the United States today after announcing his defection. He said Ethiopia had become a starving ''poorhouse'' choked by fear and hatred under Marxist rule.

Abebe Kebede, 44, boarded a Northwest Airlines flight for Washington, D.C. via Chicago with his wife, five children and a sister-in-law after holding a news conference.

Kebede received a master's degree in economics from Northern Illinois University in DeKalb, Ill. He worked worked for Ethiopia's national airline for seven years before becoming minister for domestic trade in 1980, and became ambassador to Japan in July 1985.

''My resignation (as ambassador) has come as a consequence of my frustration with the leadership in Ethiopia that has made the strengthening of the dictatorship of the party as its single most important mission while ignoring the realities that Ethiopia today has become a great poor house, desolate and unfed under the party's autocratic rule,'' Kebede said.

He said he was also leaving his position on the Central Committee of the Workers Party of Ethiopia.

''The effort to persuade the government to understand the vital importance of reform has failed, resources that are natural and human are wasted, the individual liberties, his ambitions and hopes, his religion, his traditional moral values, his social and family relationships challenged and stifled, and essential conditions of democracy have been destroyed because give and take of ideas, facts and experiences are choked by fear and hatred that pervades the system,'' Kebede said.

Ethiopia, which became closely allied with the Soviet Union in the late 1970s, is one of the world's poorest nations, and a worldwide famine relief effort was mounted in 1984 because of mass starvations.

Kebede said the Soviet Union, which provides considerable military support to Ethiopia, is in the country ''for their own political gains. If not for that, then I wouldn't know any other reasons because Ethiopia is poor and destitute.''

The U.S. Embassy in Tokyo first notified the Japanese on March 12 that Kebede had requested political asylum, and told Japanese officials on March 16 that the request had been granted, said Takashi Onda, director general of the Foreign Ministry's Middle East and African Affairs Bureau.

The U.S. Embassy declined comment today.

The Ethiopian government of Mengistu Haile Mariam has made no contact with Tokyo over the Kebede affair other than to designate an acting ambassador, Onda told reporters.

The Ethiopean ambassadors to France and Sweden recently defected, and in October, the former foreign minister, Goshu Walde, sought political asylum in the United States.