A device for calculating differences includes a difference circuit for generating difference signals .DELTA.x.sub.j =x.sub.j -x.sub.i, .DELTA.y.sub.j =y.sub.j -y.sub.i, and .DELTA.z.sub.j =z.sub.j -z.sub.i between coordinates of i having (x.sub.i, y.sub.i, z.sub.i) coordinate signals and coordinates of j having (x.sub.j, y.sub.j, z.sub.j) coordinate signals in an orthogonal coordinate system. The difference circuit includes an x-axis circuit, responsive to the x.sub.i and x.sub.j signals having a first circuit for receiving the x.sub.i coordinate signal and the x.sub.j coordinate signal and generating the .DELTA.x.sub.j ; a comparison circuit for comparing the x.sub.i and x.sub.j signals and determining whether the .DELTA.x.sub.j is less than a first set value -L.sub.x /2 corresponding to a length of a side of a virtual rectangular parallelepiped or greater than a second set value L.sub.x /2 corresponding to the length of the side of the virtual rectangular parallelepiped, L.sub.x being a value indicating the length of an elongated side in the x-axis direction of the virtual rectangular parallelepiped; an adder circuit for receiving the L.sub.x and .DELTA.x.sub.j and adding the L.sub.x to .DELTA.x.sub.j when .DELTA.x.sub.j is less than -L.sub.x /2; and a subtraction circuit for receiving the L.sub.x and .DELTA.x.sub.j and subtracting L.sub.x from .DELTA.x.sub.j when .DELTA.x.sub.j is greater than L.sub.x /2. The difference circuit includes y-axis and z-axis circuits similar to the x-axis circuit.
