[Object] To provide a sound processing device capable of effectively reducing ambient noise at low cost.[Solution] Provided is a sound processing device including: a first sound collector configured to collect a first noise signal from a noise source of noise leaking into a casing mounted to a user's ear; a first signal processing unit configured to form a first noise reduction signal used to reduce noise at a cancellation point on the basis of the first noise signal; a second signal processing unit configured to form a second noise reduction signal used to reduce noise at a cancellation point with respect to a first pseudo noise signal; an adder configured to add the first noise reduction signal and the second noise reduction signal; and a sound emitter configured to emit an output of the adder into the casing as sound. The first pseudo noise signal is a signal obtained by subtracting an output of the adder applied with a simulation transfer characteristic from an output of the first sound collector, the simulation transfer characteristic being obtained by simulating a transfer characteristic from the sound emitter to the first sound collector.