To realize a signal conversion device which is small in size and moreover does not need complicated adjustment without using a bandpass filter.

Two input signals I and Q, whose phases cross orthogonally to each other, are inputted to a first multiplier 21 and a second multiplier 22 and multiplied by a tangent value tan α of a desired phase, the signal I is added to the output of the second multiplier 22, and an output of the first multiplier 21 is subtracted from the signal Q. A signal S, having the frequency of the difference or sum between the frequencies of the input signals I and Q and the frequency of a first carrier signal C and a desired phase α is obtained, by multiplying an addition output A1 by the first carrier signal C of a prescribed frequency, multiplying a subtraction output B1 by a second carrier signal C' whose phase is different from that of the first carrier signal C by 90°, performing, by a synthesizer 28, an additive or subtractive synthesis of those multiplication results A2 and B2, and multiplying the synthesized output J by a cosine value cos α of a desired phase.