PURPOSE: To exactly display a remaining life by calculating and displaying the remaining life from practical working hours correcting the accumulated value of electrifying hours for a main power source detected by a main power source hour meter with the accumulated value of working hours for respective parts detected by an each part hour meter.

CONSTITUTION: A main power source hour meter 10 detects time from the activation of a drying device to the stop. However, the standstill hours of the respective parts from the activation to the stop detected by the main power source meter 10 are corrected by the hours, which are detected by each part hour meters 11-13, and the practical working hours of the drying device are exactly detected. Thus, from useful life hours set in advance, the remaining useful life hours. Namely, the remaining life is correctly detected and displayed.