PURPOSE: To easily detect the concentration of total evaporation residue of boiler water by a method wherein an entire-blowdown time is indicated or signaled when water level electrodes detect the normal water level and an electrode in an entire-blowdown water level detecting well detects the absence of water during steam generation of a boiler.

CONSTITUTION: An entire-blowdown water level detecting well 2 is connected to an upper header 4 at the upper end and a lower header 7 at the lower end. Two or more electrodes are furnished in the detecting well 2 and one of them is used as an entire-blowdown electrode 6. When the entire-blowdown electrode 6 detects the absence of water and electrodes in a water level detecting well 1 detect the normal water level while a boiler is generating steam, a controller 10 decides it is the time for an entire blowdown and indicates the entire- blowdown time or outputs a signal of the entire-blowdown time. Thereby, when the total evaporation residue of boiler water begins to rise, it is easily decided according to the water level detected by the entire-blowdown electrode 6.