A signal processing apparatus, comprising: an A/D, analog to digital, converter converting an inphase frequency signal and a quadrature frequency signal into amplitude sampling data including digital inphase data and digital quadrature data; a phase shifter including a phase shift lookup table that converts all amplitude sampling data outputtable from the A/D converter into phase values and approximates the phase values into first approximate integers, and thereafter, converts the first approximate integers into phase data of predetermined bits and stores the phase data, and converting the amplitude sampling data inputted from the A/D converter into the phase data by referring to the phase shift lookup table; a phase difference detector receiving the inphase frequency signal and the quadrature frequency signal inputted into the A/D converter and outputting the phase difference between the inphase frequency signal and the quadrature frequency signal; a second A/D converter converting and outputting the phase difference outputted from the phase difference detector into a digital phase difference; and a phase error lookup table storing a phase error depending on the digital phase difference outputted from the second A/D converter, a phase data memory storing the phase data outputted from the phase shifter wherein the phase data stored in the phase data memory includes the phase error; an amplitude converter including an amplitude conversion lookup table that converts all phase data outputtable from the phase shifter into amplitude values and approximates the amplitude values into second approximate integers, and converts the second approximate integers into amplitude data including digital inphase data and digital quadrature data and stores the amplitude data, and converting the phase data stored in the phase data memory into amplitude data by referring to the amplitude conversion lookup table; and a D/A converter converting the amplitude data outputted from the amplitude converter into an inphase frequency signal and a quadrature frequency signal. an A/D, analog to digital, converter converting an inphase frequency signal and a quadrature frequency signal into amplitude sampling data including digital inphase data and digital quadrature data; a phase shifter including a phase shift lookup table that converts all amplitude sampling data outputtable from the A/D converter into phase values and approximates the phase values into first approximate integers, and thereafter, converts the first approximate integers into phase data of predetermined bits and stores the phase data, and converting the amplitude sampling data inputted from the A/D converter into the phase data by referring to the phase shift lookup table; a phase difference detector receiving the inphase frequency signal and the quadrature frequency signal inputted into the A/D converter and outputting the phase difference between the inphase frequency signal and the quadrature frequency signal; a second A/D converter converting and outputting the phase difference outputted from the phase difference detector into a digital phase difference; and a phase error lookup table storing a phase error depending on the digital phase difference outputted from the second A/D converter, a phase data memory storing the phase data outputted from the phase shifter wherein the phase data stored in the phase data memory includes the phase error; an amplitude converter including an amplitude conversion lookup table that converts all phase data outputtable from the phase shifter into amplitude values and approximates the amplitude values into second approximate integers, and converts the second approximate integers into amplitude data including digital inphase data and digital quadrature data and stores the amplitude data, and converting the phase data stored in the phase data memory into amplitude data by referring to the amplitude conversion lookup table; and a D/A converter converting the amplitude data outputted from the amplitude converter into an inphase frequency signal and a quadrature frequency signal.