An observation signal processing apparatus transmits a pulse signal as a search signal, generates an observation value based on a reflected signal against a target and a delay modulation pulse signal, and performs coherent integration on the observation value to output an integration value. The apparatus includes a section for determining a coherent integration count, a section for transmitting pulse signals equivalent to the coherent integration count, a section for calculating a phase correction amount based on an estimated relative speed, and a section for performing phase-weighted coherent integration on observation values for the number of times equivalent to the coherent integration count based on the phase correction amount.