A door opening/closing mechanism fitted on a door that closes and opens an opening formed in the body of an apparatus by being brought into and out of contact with the rim of the opening has cam mechanisms that permit the door to engage with and disengage from the body at either of the right and left sides of the door. The cam mechanisms can be brought into a first lock position in which they lie symmetrically at both sides of the door and into a second lock position in which they lie symmetrically at both sides of the door, and each have a hinge pin that serves as a rotation axis in the second lock position and a groove cam that engages with the hinge pin in such a way as to be movable relative to the hinge pin. The groove cam has a slide portion on which a part of the innermost portion of the hinge pin slides when the cam mechanism is moved from the first lock position to the second lock position and of which the length in the direction of the width of the door is greater than the maximum permissible variation in the outermost distance (L) between the two groove cams formed at both sides of the door. When the door is closed, the cam mechanisms at both sides are kept in the first lock position and, when the door is opened at one side, the door slides and thereby causes the cam mechanism at the other side to be brought into the second lock position so as to be rotatably locked in the second lock position.