An apparatus for opening and closing a breech block includes a cam follower, a housing, a first plunger, a rotation unit, a pressurizing unit, an open cam, a first elastic member, and a second plunger. In the present invention, in a state that the open cam is out of the range of a moving path of the cam follower so as to restrict rotation of the cam follower, the second plunger is inserted into the housing so as to support the pressurizing unit. This may allow a closed state of the breech block to be maintained after firing. Furthermore, in the present invention, a closed mode of the breech block may be converted into an open mode, through simple manipulations, e.g., by rotating the rotation unit, i.e., by moving the pressurizing unit.