Resonators generate resonance sound data of given resonant frequencies based on sound signal data corresponding to a pitch of a key. Each resonator includes a delay circuit which delays an input based on delay time data, a second adder which adds an output from the delay circuit to input sound signal data, and a low-pass filter which performs filtering depending on a filter control signal on the output from the second adder. An output from the low-pass filter is input to the delay circuit, and a first adder adds outputs from the resonators. A storage device stores a coefficient table containing items of delay time data to be provided to the resonators, and the items of the delay time data do not match with frequencies corresponding to pitches of keys.