The purity of a liquid chemical agent is improved simultaneously with obtaining a homogeneous concentration of a specific chemical in the agent, thereby improving the quality of substrate processing steps. Pure water is supplied from a pure water supply (30) into a process tank (1) by opening an opening/closing valve (33) on a pure water supply system (7) and by closing an opening/closing valve (19) on a circulation system (5). After the process tank (1) is completely charged with pure water, the opening/closing valve (33) is closed while the opening/closing valve (19) is opened. This closes a pipe conduit (11) of the circulation system (5) and allows pure water in the process tank (1) to be circulated through the circulation system (5) by a pump (13). A first opening/closing valve (49) on a gas supply system (9) is subsequently opened to allow hydrogen chloride gas from a hydrogen chloride gas supply (40) to be fed into a mixing unit (17) of the circulation system (5). This results in mixing of the hydrogen chloride gas with pure water in the mixing unit (17) to make a liquid chemical agent.