To prevent a chemical solution from flowing back to a tap water side by reliably carrying out switching between a supply circuit of the chemical solution to be used when sterilizing a fluid circuit in a medical unit using the chemical solution, and a tap water supply circuit to be used when treatment is performed.

A fluid circuit switching device 12 includes steps of: temporarily separating a water supply source 2 and a fluid supply source in a fluid tank 11 side in a non-contact state; and connecting either one of the fluid circuit to the fluid circuits in the medical unit after going through the step of the separation.