To provide a control board assembly of a game machine that prevents an illicit ROM from being mounted and that enables easy confirmation of conditions in a connection member-side open space.

A main control board assembly 50 of a Pachinko game machine includes: a main control board comprising a fixed ROM and a removable ROM 110 mounted thereon; and a base case for housing and retaining the main control board. The control board has a socket connector 120 mounted thereon for electrically connecting the removable ROM 110 to the main control board. The connection member-side open space 124 is provided between the socket connector 120, the removable ROM 110 and the main control board. The main control board has a protrusion passing hole 51c formed in a portion opposite to the connection member-side open space 124. The base case has a storage-regulating protrusion 66 inserted through the protrusion passing hole 51c and protruding in the connection member-side open space 124.