A mixing apparatus (1; 201; 301) comprising: a production unit configured to produce a working fluid that is in a supercritical state or a subcritical state; a storage unit configured to store a material; a dissolving unit (30; 230) into which the working fluid and the material are fed, the dissolving unit (30; 230) configured to dissolve the material in the working fluid; a mixer (50; 250) configured to mix the material together in the presence of the working fluid; a working fluid inflow valve configured to open or close a flow passage through which the working fluid is to pass to flow into the dissolving unit (30; 230) from the production unit; and a material feed valve configured to open or close a flow passage through which the material is to pass to be fed from the storage unit into the dissolving unit (30; 230), characterized by a mixer inflow valve configured to open or close a flow passage through which the working fluid and the material are to pass to flow into the mixer (50; 250) from the dissolving unit (30; 230); a branch flow passage that is a passage through which the working fluid produced in the production unit is to flow, that branches off the production unit on the upstream side of the working fluid inflow valve, that merges into the flow passage through which the working fluid and the material are to pass to flow into the mixer (50; 250) from the dissolving unit (30; 230) at a position on the upstream side of the mixer (50; 250), and that merges into the mixer (50; 250) at a position on the downstream side of the mixer inflow valve; and a branch flow passage opening/closing valve configured to open or close the branch flow passage. a production unit configured to produce a working fluid that is in a supercritical state or a subcritical state; a storage unit configured to store a material; a dissolving unit (30; 230) into which the working fluid and the material are fed, the dissolving unit (30; 230) configured to dissolve the material in the working fluid; a mixer (50; 250) configured to mix the material together in the presence of the working fluid; a working fluid inflow valve configured to open or close a flow passage through which the working fluid is to pass to flow into the dissolving unit (30; 230) from the production unit; and a material feed valve configured to open or close a flow passage through which the material is to pass to be fed from the storage unit into the dissolving unit (30; 230), characterized by a mixer inflow valve configured to open or close a flow passage through which the working fluid and the material are to pass to flow into the mixer (50; 250) from the dissolving unit (30; 230); a branch flow passage that is a passage through which the working fluid produced in the production unit is to flow, that branches off the production unit on the upstream side of the working fluid inflow valve, that merges into the flow passage through which the working fluid and the material are to pass to flow into the mixer (50; 250) from the dissolving unit (30; 230) at a position on the upstream side of the mixer (50; 250), and that merges into the mixer (50; 250) at a position on the downstream side of the mixer inflow valve; and a branch flow passage opening/closing valve configured to open or close the branch flow passage.