PURPOSE: To shorten washing time, by feeding a continuously fed liquid to the main flow path of an injector, and continuously feeding a color forming liquid out of a nozzle protruded to the upstream direction from the position of the downstream bent flow path, into said liquid.

CONSTITUTION: A liquid fed continuously from a water-purifying device 1, i.e. washing soln. is supplied into the main flow path (m) of an injector 3 made of glass while flow rate is adjusted with a rotor meter 2, and a color forming soln. is continuously injected from a tank 5 with a pump 4 from a nozzle (n) protruded upstream from the position of the straight flow path (s) downstream from the position (m) through a bent path. The process of a measuring device consists of the first step of continuously injecting the color forming soln. mentioned above, and the second step of mixing the color forming soln. added into the washing soln. by passing through a tubular mixer 6 packed with resistors (b), etc., thus permitting enhancement of responsiveness and accuracy, and reduction of washing time.