There is provided a procedure for generating tone-generation controlling key-on and key-off signals, which is applicable to either a first-type key operation detection device that, in response to depressing operation of a key, generates detection signals in correspondence with at least predetermined upper and lower positions or a second-type key operation detection device that, in response to depressing operation of a key, not only generates detection signals in correspondence with at least predetermined upper and lower positions but also generates a detection signal of an intermediate position unobtainable by the first-type key operation detection device. On the basis of the detection signals received from the key operation detection device applied, a first key-on signal is generated when a key has reached the lower position from the upper position. Further, on the basis of the detection signals, a determination is made as to whether or not particular operation has been performed for causing the key to reach the lower position from the intermediate position after generation of the first key-on signal but before generation of a key-off signal, and, if so, a second key-on signal is generated. Furthermore, on the basis of the received detection signals, a key-off signal is generated in correspondence with the generated first or second key-on signal when the key has returned to a predetermined key-off position after generation of the first or second key-on signal.