To provide an arithmetic method having high possibility of shortening the arithmetic time comparing with a method using a conventionally employed ATM (Acoustic Transfer Matrix) and MATM (Modal Acoustic Transfer Matrix) in a device or the like for determining sound pressure or the like based on the oscillating rate of the surface of an oscillating body.

This radiation power arithmetic device calculates a sound pressure mode vector based on the oscillating rate of the surface of the oscillating body, determines the radiation impedance matrix, calculates Hermite component, and calculates the radiation power. This radiation power arithmetic device calculates a sound pressure mode vector based on the oscillating rate, calculates the matrix, calculates the mode expansion coefficient, calculates the oscillating rate distribution, calculates the sound pressure distribution, and calculates the sound pressure. This radiation power arithmetic device calculates particle velocity based on the calculation result of the sound pressure distribution.