PURPOSE: To decrease the interpolation arithmetic frequency in the case the interpolation is carried out among plural vector pairs for acquisition of the prescribed data by providing a vector holding part, a vector extracting part, a rotary matrix generating part, a rotation processing part, and an interpolation arithmetic part.

CONSTITUTION: A vector holding part 1 holds the data on a normal line vector pair to be interpolated, the data on a light source vector pair to be interpolated, and the data on a visual line vector pair to be interpolated respectively. A vector extracting part 2 extracts the data on the visual line vector pair out of the vector data held by the part 1. These two vector data are set in parallel with each other after a rotary matrix is produced via a rotary matrix generating part 3. Then a rotation processing part 4 multiplies the normal line vector, the light source vector, and the visual line vector by the rotary matrix. An interpolation arithmetic part 5 performs the interpolation arithmetic for interpolation among the original normal line vector, the light source vector, and the normal line vector and the light source vector undergone the rotation processes respectively. As a result, the number of vector pairs to be interpolated can be decreased together with reduction of the required time.