To provide an opening/closing mechanism opening a door in either of the forward and backward directions and surely locking a door closed position.

A lock member D holding the door closed position is arranged in the end face on the free end side of the door A and constructed of a cylindrical case having an opening part in one end part and a latch 6 housed in the case freely retractably, always energized in the projecting direction from the back part by means of a spring 3 and provided with an engagement projection part 5 to be engaged in an engagement recess part 4 in an engaged member E arranged in a fixed frame B facing the front end. In a plane view, the engagement projection part 5 is provided with a triangular tip part and laterally parallel side face engagement parts 5b formed in the slant base part of the triangular tip part, while the engaged member E is provided with a guide piece 8 abutting on the side face engagement part 5b of the latch in closing the door so as to retract the latch 6, and push buttons 7 retracting the latch to an unlocking position are arranged on the front and back faces of the door.