Each of a plurality of injection molding machines is provided with clamp plates fixed on platens and a plurality of magnet units to generate magnetic force for fixing a mold to the clamp plate fitted on the clamp plates; and the plurality of injection molding machines are provided with a plurality of common connectors having the same structure, each of which is electrically connected to the magnetic unit, a molding machine control unit and a power source device; and the mold fixing system comprises a set of operation control units to be shared by the plurality of injection molding machines, and when a mold is to be exchanged in any of the injection molding machines, the plurality of magnet units are switched to the non-adsorbing state or the adsorbing state via the operation control unit in the state where the operation control unit is connected to the common connector of the injection molding machine.