To surely prevent a freezing trouble in a cold district by attaching a switch for discharge valve forcible opening for forcibly opening a discharge valve and forcibly discharging water remaining in a washing rotary tank after automatic waste water discharge is completed to a washing machine.

On the upper rear part of a trailer truck, four fully automatic washing and dehydrating machines 6 for instance are respectively loaded. The respective fully automatic washing and dehydrating machines 6 are respectively provided with the switch 6a for the discharge valve forcible opening. By turning ON the switch 6a, a water amount sensor 6e detects the water remaining in the washing rotary tank 6b even after the automatic waste water discharge is completed, the discharge valve 6c is forcibly opened and waste water is forcibly discharged. Thus, the freezing trouble in the cold district is prevented. An electric circuit incapable of starting washing when the switch 6a for the discharge valve forcible opening is ON for safety is constituted.