PURPOSE: To improve sensing and detecting precision of foreign matter in a process for detecting the foreign matter between a fixed upper mold and a movable lower mold.

CONSTITUTION: Clamping pressure of a fixed upper mold 107 and a movable lower mold 114 and pushing pressure for ejector plates 125, 126 for releasing resin molded piece mounted respectively on the upper and lower molds 107, 114 are set in such a manner that they do not interfere with each other, so as to assuredly prevent the pushing pressure to be exerted on the ejector plates 125, 126 from working as a load to restrict the clamping pressure of the upper and lower molds 107, 114. Accordingly, in the mold clamping process of the upper and lower molds 107 and 114, the presence of foreign matter between the upper and lower molds can be sensed and detected assuredly in accordance with the presence of load for restricting the clamping pressure.