A power supply connected to an ultrasonic treatment instrument. The power supply includes an oscillator, a field programmable gate array (FPGA), and a drive circuit. The oscillator generates a basic clock. The FPGA receives the basic clock generated by the oscillator. The FPGA generates a pulse width modulation (PWM) wave for driving a ultrasonic transducer based on the basic clock. The drive circuit is connected to the FPGA, and digitally amplifies the PWM wave to output the amplified PWM wave. The FPGA outputs the PWM wave when an output signal for driving the ultrasonic treatment instrument is input, and the PWM wave is modulated by comparing a value of first data representing a sine wave in constant current control with a value of second data representing a reference wave. The FPGA outputs a rectangular wave with a predetermined duty ratio when the output signal is not input.