PURPOSE: To eliminate the influence of a power source and temperature and to stabilize oscillation by providing differential amplifiers which compare the potential of a capacitor with a reference potential, differential switches which switch the charging and discharging of the capacitor by its comparison output, and differential switches which switch the reference potential.

CONSTITUTION: The 1st differential switches Q1 and Q2, and the 2nd differential switches Q3 and Q4 operate so that while the transistor Q1 turns on, the TRQ3 turns on, so while the capacitor C is discharged, differential amplifiers Q5 and Q6 compare a low-level reference potential (1st potential) with the capacitor potential and maintain that while the capacitor potential is greater than the 1st potential. When the capacitor potential becomes lower than the 1st potential, the 1st and the 2nd differential switches are inverted to charge the capacitor while raising the reference potential up to the 2nd potential, thereby maintaining this state, until the capacitor potential rises up to the 2nd potential. Then, the gains of the differential amplifiers are increased to reduce the offset potentials of the switches.