A triangular wave generating circuit adapted to a class-D amplifier is designed not to use a PLL circuit and to secure robustness regarding an amplification gain irrespective of variations of voltages, thus producing a high-quality triangular wave with a simple circuit constitution. First and second constant currents, which are generated in proportion to positive and negative voltages, are alternately and periodically selected using high impedance elements without causing noise. A first integrator produces a triangular wave in response to charged electricity realized by the first and second constant currents, wherein the triangular wave is supplied to a second integrator performing servo-amplification operation so as to suppress phase shifts thereof. Hence, it is possible to maintain a constant gain for the class-D amplifier irrespective of variations of voltages since the maximal and minimal voltages values of the triangular wave are made proportional to the positive and negative voltages.