To provide a substrate treatment device for precisely controlling a replenishing rate of a diluted solution by employing a structure which enables linear adjustment of a flow rate.

An output pressure Pout of an electropneumatic converter 39 is controlled in accordance with the density detected by a density detector 45 to operate an air control valve 37 and thus to control the replenishing rate of pure water. The electropneumatic converter 39 is capable of linearly varying the output pressure Pout in response to an input signal Sin from a density controller 55. By operating the air control valve 37 in accordance with the variation, the replenishing rate of pure water can be linearly controlled to the order of the output pressure Pout. Thus, pure water can be replenished with precision in accordance with the density detected by the density detector 45.