PURPOSE: To speed up the molding work by accomplishing various checking works for the mold used in the injection molding and the maintenance and inspection works prior to the molding in a molding preparator for preheating of the mold.

CONSTITUTION: A heat medium fluid (50°C) adjusted in the temperature is circulated through a passage in a mold to be preheated. Then, current flows through a heater such as hot runner and hot gate to perform a preheating up to 200°C. At this point, the value of the current flowing through a heater circuit is measured with an ammeter to check the circuit for disconnection. In addition, by the operation of a holding changeover switch, two molds are opened and closed with a sliding cylinder to check for the matching thereof. Here, a hydraulic actuator driving test, the operation of a pneumatic acutator circuit and the like are also ascertained. This enables the accomplishing of the ascertaining in the operation for the molds and the maintenance and inspection work prior to the molding during the preheating of the die thereby eliminating decrease in the operating rate of an injection molding machine.