PURPOSE: To provide a QPSK modulating device whose circuit configuration can be simplified, and phase noise characteristic can be satisfactory by constituting a first local oscillator of a frequency synthesizer whose step size is small.

CONSTITUTION: The synthesizer whose oscillation frequency can be changed by a fine step is used for a first local oscillator 2. Thus, a modulating frequency is not deteriorated much even when the oscillation frequency changes by almost 1MHz, and a variable frequency width is ±500kHz and a step is 25kHz. Therefore, when the oscillation frequency of a second local oscillator 5 is 340 MHz±40MHz/step 1MHz, a desired intermediate frequency signal can be obtained. Thus, the number of frequency-division can be decreased, and the phase noise characteristic can be improved. Also, in a QPSK demodulating device, a frequency synthesizer whose step size is minute is used for a reference signal reproducer similarly to the first local oscillator 2.