PURPOSE: To enable a positive type resist developer to be adjusted in specific concentration by a method wherein the liquid quantities in a mixing vessel is measured by a load cell for manipulating respective equipments, control valves, etc., using a sequence controller by an ultrasonic measuring device.

CONSTITUTION: Raw developer is fed to a mixing vessel 3 from a raw developer vessel 1 while superpure water is fed to the same 3 through a piping 6. Next, respective liquid quantities fed to the mixing vessel 3 are measured by a load cell 8 so that respective control valves and a pump may be controlled rapidly and precisely through a sequence controller 9. At this time, the liquid in the mixing vessel 3 is mixed for specific time using a circulating water pump 10 while controlling the circulating liquid quantity by a control valve 11. Next, a part of the mixed solution is fed to an ultrasonic propagation rate measuring device 12 so as to measure the concentration of the mixed solution with high precision by a temperature compensated ultrasonic concentration meter. Furthermore, the raw developer or superpure water is to be added according to the measured values to produce the developer in the objective concentration.