In a temperature control device which maintains an object to be controlled at a preset constant temperature and an arrayed waveguide grating optical wavelength multiplexer/demultiplexer, the adverse effect of the junction temperature of the semiconductor control element is eliminated for the purpose of simplifying the overall structure and improving operation efficiency. The object to be controlled and semiconductor control element are fixed on the soaking plate. A negative feedback control is applied to the conduction condition of the semiconductor control element. Consequently, the junction temperature of the semiconductor control element is effectively used as a heating source not as a loss, thereby reducing the power consumption and stabilizing the performance.