The calculating device has first and second Fourier transform portions performing Fourier transforms on two respective inputted signals; a complex conjugate deriving portion deriving a complex conjugate of a value outputted from the first Fourier transform portion; a multiplier multiplying the complex conjugate with the value outputted from the second Fourier transform portion, and outputting the value after the multiplication; a Hilbert transform portion performing a Hilbert transform on the value after multiplication; first and second inverse Fourier transform portions performing respective inverse Fourier transforms on the value after multiplication and the value outputted from the Hilbert transform portion; a phase relationship deriving portion deriving a phase relationship between the two inputted signals based on the values outputted from the first and second inverse Fourier transform portions; and a time difference deriving portion deriving a time difference between the two inputted signals based on the phase relationship.