To secure appropriate preheating time without giving dissatisfaction with time prolongation accompanying the preheating to a customer going to purchase a commodity by the use of an automatic vending machine by executing preheating in parallel to the execution of the function of the automatic vending machine.

When the customer starts a purchasing operation by the automatic vending machine and first supplied coin is detected, the supplied coin is discriminated and a second switch 6 is turned on. When the second switch 6 is turned on, the preheating of a magnetron 5 is started. A preheating timer where appropriate preheating time is previously set is started. When the preheating timer is timed up, a first switch 2 is turned on and a change-over switch 7 is operated after prescribed seconds which is previously set so as to switch a system to a contact point 7b-side. Thus, the magnetron 5 oscillates, a microwave oven starts preheating and a heating timer where appropriate heating time is previously set starts.