PURPOSE: To enable the detection of molten metal pressure at the thick part being difficult-to-solidify and to accurately detect only the molten metal pressure.

CONSTITUTION: To a pressurizing means 20 having a piston 23 for pushing a pressurizing pin 18 in a cavity 17 direction with hydraulic pressure, a molten metal pressure detecting liquid chamber 26b, in which the molten metal pressure transferred to the piston 23 through the pressurizing pin 18 acts, is arranged. From the liquid pressure in the molten metal pressure detecting liquid chamber 26b detected by a pressure detecting means 51, the molten metal pressure in the cavity 17 is calculated by a control means 61. It is discriminated by the control means 61 whether this calculated molten metal pressure is in the permissible range or not.