Provided are a three-way valve for flow rate control and a temperature control device using the three-way valve for flow rate control. The three-way valve for flow rate control is capable of controlling a mixture ratio between two kinds of fluids with higher accuracy, as compared to a three-way valve including an inflow port which allows inflow of a high-temperature heat-medium circulating liquid, an inflow port which allows inflow of a low-temperature heat-medium circulating liquid, an outflow port which allows outflow of a constant-temperature heat-medium circulating liquid, and a control valve configured to control the flow rate ratio between the high-temperature heat-medium circulating liquid and the low-temperature heat-medium circulating liquid. The three-way valve for flow rate control includes: a valve main body including a valve seat, the valve seat having a columnar space and having a first valve port, which allows inflow of a first fluid and has a rectangular cross section, and a second valve port, which allows inflow of a second fluid and has a rectangular cross section; a valve body being provided in a freely rotatable manner in the valve seat of the valve main body so as to simultaneously switch the first valve port from an closed state to an opened state and switch the second valve port from an opened state to a closed state, the valve body being formed into a half-cylindrical shape having a predetermined central angle and being formed into a curved-surface shape at each of both end surfaces of the valve body in a circumferential direction; and drive means for driving the valve body to rotate.