A signal processing device is composed of a signal acquisition unit and a signal processing unit. The signal acquisition unit acquires a signal corresponding to a vibration propagated from a string attached to a stringed instrument from a pickup element that picks up the signal corresponding to the vibration. The signal processing unit includes a filter that performs convolution operation using a filter coefficient set in the filter, the signal processing unit applying the convolution operation to the acquired signal through the filter and outputting a processed signal. The filter is set with the filter coefficient corresponding to a transfer function which has a frequency response developing a plurality of peak waveforms corresponding to resonance of a body of another stringed instrument different from the stringed instrument within a specific frequency range and which allows components of the peak waveforms to decay more rapidly than a component of a fundamental sound in the vibration of the string in the processed signal.