This paper deals with the design of instruments for measurement of the strength of the swing-stroke in piano playing. The strength can be measured by the velocity of the hammer. This is easily detected by a photo-sensitive transistor, and is suitable for digital electronic notation. The velocity is converted to the time interval by photo-transistors and flip-flop. The signal opens a gate, through which flows a clock pulse of 100KHz. This is transmitted to an electronic, pulse counter consisting of 4 decimal counting units. The number of pulses counted by this counter is proportional to the inverse of the velocity, and is compared with 5 present values to activate one of 6 magnetic counters. Each magnetic counter indicates the number of the swing-stroke, of which the velocity belongs to a previously calssified group. This equipment makes it possible to measure the intensity of the tone and its distribution precisely.