To prevent error action and troubles caused by stick of threads and to improve measure accuracy of detergent concentration by arranging a detergent concentration sensor that is equipped with coaxially arranged cylindrical inner electrode in inner side of a cylindrical outer electrode having a plurality of penetrating holes in its wall, in flow path of cleaning liquid.

A detergent concentration sensor 1 is formed as a thin cylindrical shape and is arranged in a pipeline through which a cleaning liquid passes. The sensor 1 has an outer electrode 2 installed in front end of a holder 10 to which a coaxial cable 22 is connected through a cord bush 21, and the other end of the coaxial cable 22 is connected to an input part of the measuring device of detergent concentration. The outer electrode 2 is formed as a cylindrical shape and a plurality of penetrating holes 3 are formed in its pipe wall, and inner electrode 5 is coaxially arranged in its inner side at intervals. While an electric current passes through, an electric potential difference occurs and a current in correspondence with detergent concentration of the cleaning liquid occurs, so the detergent concentration is measured according to this current by the detergent concentration meter.