A musical tone signal processing apparatus configured to extract musical tone signals that are signal processed for a plurality of localizations. Such an apparatus may be configured to carry out signal processing for signals that have been extracted by first retrieving processing (S100) and/or second retrieving processing (S200). The first retrieving processing (S100) and the second retrieving processing (S200) extracts a musical tone signal (e.g., the left channel signal and the right channel signal) that satisfies each of the conditions that have been set (e.g., frequency, localization, and maximum level) as the extraction signal. Accordingly, the extraction signal can be extracted to allow the musical tone signal processing apparatus to signal process the extraction signal for each of the plurality of conditions.