A signal processor is configured to perform a process equivalent to performing a series of fixed-to-rotating coordinate conversion, a predetermined process and then rotating-to-fixed coordinate conversion, while maintaining linearity and time-invariance. The signal processor performs a process given by the following matrix G: G = [ F ⁡ ( s + j ⁢ ⁢ ω 0 ) + F ⁡ ( s - j ⁢ ⁢ ω 0 ) 2 F ⁡ ( s + j ⁢ ⁢ ω 0 ) - F ⁡ ( s - j ⁢ ⁢ ω 0 ) 2 ⁢ j - F ⁡ ( s + j ⁢ ⁢ ω 0 ) - F ⁡ ( s - j ⁢ ⁢ ω 0 ) 2 ⁢ j F ⁡ ( s + j ⁢ ⁢ ω 0 ) + F ⁡ ( s - j ⁢ ⁢ ω 0 ) 2 ] where F(s) is a transfer function representing the predetermined process, ω0 is a predetermined angular frequency and j is the imaginary unit.