PURPOSE: To suppress production of spurious radiation of an output signal by combining two output buffers with a different drive capability, selecting an output buffer with a smaller driving capability when a power voltage rises and selecting an output buffer with a larger driving capability when a power voltage drops from a prescribed value.

CONSTITUTION: Two output buffers 5, 6 with different driving capability are combined and when a power voltage 3 is fluctuated in a higher level from a prescribed value, the output buffer 6 with smaller driving capability is operated and when the power voltage 3 is fluctuated in a lower direction than the prescribed value, the output buffer 5 with large driving capability is operated. Thus, the current flowing to the output buffers 5, 6 is suppressed depending whether the power voltage is fluctuated in the higher level than the prescribed value or lower level than that and undesired radiation from the output signal caused when the power voltage 3 is fluctuated in a higher level than a required level is suppressed.