To provide a mixing apparatus for a treating liquid in which a mixing tank storing the treating liquid can be small-sized in the case of preparing the treating liquid of a low hydrofluoric acid density.

The mixing apparatus comprises: a liquid chemical supply pipe for supplying hydrofluoric acid to the mixing tank; a diluent supply pipe for supplying pure water to the mixing tank; and a supply control pump 6 for controlling the supply amount of the hydrofluoric acid to be supplied from the liquid chemical supply pipe to the mixing tank. The supply control pump is constituted of: a main body 16 where a liquid chemical chamber 17 for storing the hydrofluoric acid supplied from the liquid chemical supply pipe is formed; a diaphragm 18 provided while making one side surface face the liquid chemical chamber; a piezoelectric element 21 provided on the other side face of the diaphragm for vibrating the diaphragm; and a nozzle plate 19 where a nozzle hole 19a is formed and which is provided facing the diaphragm across the liquid chemical chamber for discharging the hydrofluoric acid stored in the liquid chemical chamber from the nozzle hole by the vibration of the diaphragm.