The player piano provides a string-striking mechanism including a hammer driven by corresponding one of eighty-eight keys, and a photo sensor is provided in the vicinity of and also positioned to face with the predetermined member of this string-striking mechanism to be rotated responsive to a key-depression. This photo sensor outputs a signal which is varied in response to a distance of the predetermined member therefrom, so that the output thereof may have a peak value. Based on the peak value, first and second output values are respectively calculated, wherein these values corresponding to the predetermined two measuring points arranged between the photo sensor and the predetermined member are outputted from the photo sensor before and after the peak value respectively. On the basis of the distance between two measuring points and passing time to be passed when the sensor output is varied from the first output value to the second output value, a string-striking velocity of said string-striking mechanism representing a performance state of the play piano is to be computed. Thus, fixation and adjustment of the sensor can be made easily, thereby the string-striking velocity of the hammer can be sensed accurately.