PURPOSE: To conduct correct molding throughout fine parts of a molding surface by the use of a mold and a mold clamping device capable of adjusting the volume of a mold cavity.

CONSTITUTION: Meanwhile resin is injected to a molding cavity 17, the inner part of the molding cavity is filled with resin in its initial narrow volume and thereafter, as a filled state being kept, the inner pressure is maintained at constant. Namely, a constant pressure injection is effected. The quantity of injection resin mold is measured at the side of an injection unit. When PU 26 for Nc confirms the completion of injection, it acts to switchover the control of a servo-motor 24 for mold clasping to positional control again via a servo CPU in order to advance a movable disc 2 for monitoring the output F2 of a pressure sensor 18. At the time when the pressure value F2 comes to be equal to the pressure value F1 of a set high pressure mold clamped state, control of the mold clamping servo-motor 24 is switched to control of torque again for permitting the torque limit value to be F1. The compression margin being remained by low pressure mold clamping is solved and resin within the inner part of the molding cavity 17 is compressed, to thereby maintain the state.