A sensing device is constructed by a sensor case which is attached to a bottom surface of a footwear (e.g., sole of a shoe) and which contains a piezoelectric sensor, a sensor fixing member, a disc plate pressure member and an annular elastic member. The sensor fixing member has elastic deformability so that the sensor fixing member is located to face with the disc plate pressure member with an air gap in which the sensor fixing member is capable of deforming within a limit of elasticity thereof. Thus, it is possible to perform musical tone control in response to an output of the piezoelectric sensor when a foot motion is applied to the footwear. In addition, a musical tone control apparatus of a percussion instrument type is constructed using a pad unit stored in a pad storage portion of an upper case. Herein, the pad unit is constructed by a pad skin unit and a sensing unit. The pad skin unit has a beat surface to be beaten, while the sensing unit contains a piezoelectric sensor, a sensor fixing member and a radial pressure member. Both of the sensor fixing member and the disc plate pressure member (or radial pressure member) are assembled together in such a way that center parts thereof are securely fixed to each other, so the piezoelectric sensor is capable of producing a same output in response to same external force (e.g., foot step force or beat force), regardless of directions to apply such force.