To provide a game machine adopting an opening part sealing structure capable of making illegal alteration difficult.

A projection piece 10 to be a part of a lid body and a nut holding part 20 to be a part of a case main body are formed so as to face each other at the time of mounting the lid body on the case main body. In the state of fitting an insert nut 40 while housing a steel ball 50 in the nut holding part 20, when a screw 30 for which a cylindrical wall part 32b as a bend part is formed at a tip is screwed, since the cylindrical wall part 32b is abutted to the steel ball 5 and pushed open, large torque is required at the time of loosening the screw 30. Also, since a one-way cut groove 31a is formed at the head part 31 of the screw 30, sufficient torque can not be applied and the screw 30 can not be loosened practically. Also, the steel ball 50 prevents a wall part made of resin from being easily shaven and provides the effect of preventing illegality.