PURPOSE: To accurately perform the washing of a filter medium and also to extremely inhibit the reduction of water in a bath by unnecessary backwash by constituting a filter so that backwash is automatically practiced in case filtering operation is performed at a preset time.

CONSTITUTION: A controller 17 by which backwashing operation is practiced in case filtering operation time reaches preset time at a time of filtering operation is provided. As the controller 17, an apparatus performing the following functions is used. For example, a pump 2 is drived by the turning-on operation of a filtering operation switch 14, and the driving of the pump 2, changeover of a three-way valve 5 and the opening and closing of a solenoid on-off valve 9 are performed by the turning-on operation of a backwashing operation switch 15. Furthermore by the turning-on operation of a capacity changeover switch 16, the opening of the valve 9 is performed and also in case the integrated time of a built-in timer integrating the filtering operation time of a filter 6 reaches preset time, filtering operation is automatically changed over to backwashing operation and backwash is performed at a constant time and thereafter the timer is cleared to return the filter to filtering operation.