PURPOSE: To quicken preheating by setting in advance the reference of the molten metal holding time inside a die, setting an allowable fluctuation time as well and controlling the molten metal holding time within the allowable range according to the die temp. immediately before the filling of each shot.

CONSTITUTION: A preheating control device 11 is provided for the diecast die 2 composed of a fixed die 4 and moving die 6 and the molten metal holding time is controlled according to the die temp. just before the filling. Namely the control device 11 incorporates the program having a specified flow chart and the molten metal reference holding time t0 inside the die and allowable fluctuation time Δt are set in advance. The die temp. sensor 10 inside the fixed die 4 detects the die temp. T1 of each shot just before the molten metal pouring and the control device 11 controls within the range of the allowable fluctuation time Δt the molten metal holding time (t) by comparing it with a die temp. set value T2. The die preheating time is shortened and preheating is quickened because the molten metal holding time is controlled according to the die tmep. T1.