In an ultrasonic operation apparatus according to the present invention, in order to detect the initial resonance frequency (Fro) promptly once a foot switch is pressed, under the control of a sweeping speed control circuit, the D/A converter D/A converts 8-bits of signals (phase difference amount signal .DELTA..theta.) representing the amount of the phase difference detected by the phase comparator, and the output of the D/A converter is output to a VCO so as to become a clock for a sweep circuit. When a phase difference amount between .theta.i and .theta.v is large and a drive frequency setting signal (Fs) is far from Fro, the sweeping speed for Fro detection is increased. When a phase difference amount between .theta.i and .theta.v is small and Fs is close to Fro, the sweeping speed for Fro detection is decreased.