To provide a control device for stabilizing the output property of a hydraulic solenoid valve even when used in the air.

The control device reads an output pressure command signal for controlling an automatic transmission (S1), and determines whether the output pressure command signal is to reduce output pressure (S2). If the signal is to reduce the output pressure, it determines whether a command is to reduce target command pressure as a target value down to zero. If the target command pressure is zero, it sets the target command pressure as PO(PO=0+ΔP) to be positive pressure somewhat higher than the atmospheric pressure, and outputs a control signal to the hydraulic solenoid valve to change the output pressure into target command pressure PO (S4). Then, it is returned to a main routine.