PURPOSE: To filter the volume of water corresponding to the degree of filthy water and to save energy by controlling speed of a motor driven pumps in correspondence with an extent of filthiness of water for a pool.

CONSTITUTION: An extent of filthiness of water for a pool 1 is measured, and speed of a motor 5 driving water circulating pumps 3 and 4 for the pool 1 is changed by control of a invertor 1 in correspondence with the data. Speed of the motor 5 is controlled in correspondence with daily, weekly and monthly schedules of driving of the pumps 3 and 4 and information synthesizing correction by the data measuring an extent of filthiness of water for the pool 1. After that time, in the measurement of filthiness, transparency is measured, control by the invertor 1 is made in correspondence with the data, and a substance emitting visual lights is used as a light source. In addition, as a light detector 23 of a light receiving section, a photodiode, a phototude, etc., are used.