PURPOSE: To write data into a ROM with high efficiency and with no mistake by setting a controller between the ROM of a temperature compensation oscillator and a temperature information signal generator to output the temperature information signal to the outside and writing the external signal into the ROM.

CONSTITUTION: A host computer 13 controls a thermostatic chamber 18 to a 1st temperature, and the temperature information received from a temperature sensor 1 is inputted to the computer 13 from an address register 6 via a controller 15 and a P/S converter 16. The computer 13 compares a signal code with the oscillation frequency of a VCXO 4 and at the same time outputs the signal code as a compensation code. This code is used as the control voltage of the VCXO 4 via an S/P converter 17 and the controller 15. In this case, a desired compensation code is decided by changing successively the compensation codes so that the prescribed oscillation frequency is obtained. When the compensation code that secures the prescribed oscillation frequency is obtained, a temperature code of the relevant time point is written into a ROM 7 by the controller 15 as an address. Thus one cycle is through.