A convolution integrator that can be used favorably to prepare, at high speed, a computer generated hologram that can reproduce a reproduction image formed by reproduction points at various distances and differing in initial phase is provided. A plurality of element processors PE are practically cascade-connected. Each element processor PE includes: a constant generator 91A, outputting a propagation function value that is generated based on a coordinate value Z and an initial phase value P; a multiplier 92, multiplying the propagation function value and a luminance value I and outputting the product value; an adder/subtractor 93, performing addition/subtraction on the product value and a hologram time series signal PDin and outputting the sum/difference value; and a register 94, receiving, holding, and then outputting the sum/difference value as a hologram time series signal PDout. The hologram time series signal PDout, output from a register 94 of an element processor of a preceding stage that is cascade-connected, is input as the hologram time series signal PDin into an adder/subtractor 93 of an element processor of a subsequent stage.