An apparatus for locking a tape recorder door where even when a cassette is not loaded in the recorder, foreign objects are prevented from being inserted through the tape recorder door which is blocked by a locking pin of a locking lever, so that a breakdown or an accident due to the insertion of a foreign object can be avoided. However, when the cassette is inserted, an opening pin of an opening lever is pushed by the edge of the cassette to thereby raise the locking lever which is engaged with the opening lever, thus opening the door.