The invention provides a temperature-compensated crystal oscillator employing an AT-cut crystal resonator in a crystal oscillation circuit, having a substantially constant oscillation frequency at approximately room temperature, wherein an output of a temperature sensor, having a substantially rectilinear temperature characteristic, is converted into square-law cure signals by a square-law converter circuit on the lower temperature side and a square-law converter circuit on the higher temperature side, respectively, which are then combined together by a signal synthesis circuit, producing a rectilinear correction signal. Frequency adjustment circuits are controlled by the signal, and temperature compensation is achieved by executing rectilinear correction of the oscillation frequency of the crystal oscillation circuit. In the case where the oscillation frequency of the AT-cut crystal resonator has a temperature characteristic having any gradient, a gradient is added to the rectilinear correction signal.