To produce a high precision oscillation frequency irrespective of temperature fluctuations, supply voltage fluctuations and circuit constant variations without using a reference frequency signal.

In a delivery inspection, a resistance value of a resistance 5 required to match an oscillation frequency to a target frequency is sequentially determined while changing environmental temperature and supply voltage of oscillation operation, and a data table matching the resistance value with the environmental temperature and supply voltage is stored in a memory 18. In an actual usage state of a CR oscillation circuit, a control circuit 17 A/D-converts voltages Va, Vb input from a temperature detection circuit 15 and a supply voltage detection circuit 16 at every predetermined control period. The resistance value of the resistance 5 corresponding to the voltages Va, Vb is read out from the data table stored beforehand in the memory 18, and switches 7a-7c are selected such that a resistance value R of the resistance 5 becomes equal to the specified value read out.