To reduce cost and to prevent deterioration in quadrature modulation accuracy due to an error of a phase shifter.

A digitally modulated signal source consists of an optional waveform generator 11 that generates a quadrature modulated signal expressed by I(t) cos(2πfbt)-Q(t) sin(2πfbt), where Q(t) is the quadrature component, t is time, I(t) is the in-phase component and fb is a carrier frequency, a 1st carrier generator 12 that generates a sine wave whose frequency is fc (fc>fb), a 1st mixer 13 that mixes an output of the 1st carrier generator 12 with the output of the optional waveform generator 11, a 1st band pass filter 16 that receives the output of the 1st mixer 13 and selects only the upper side band of the output, a 2nd carrier generator 15 that generates a sine wave whose frequency is fd (fd>fb+fc), a 2nd mixer 16 that mixes the output of the 2nd carrier generator 15 with the output of the 1st band pass filter 15, and a 2nd band pass filter 17 that receives the output of the 2nd mixer 16 and selects only the upper side band of the output.