In the Diesel engine with an open combustion chamber, the utilization of gas motion produced in the chamber is important to improve engine performance. As the first step to finding the relation between the shape of combustion chamber and the formation process of squish, squish velocity has been measured with a velocity detector of solenoid type by motoring a model engine, and then investigated theoretically. The measured squish velocity is not much different from the ideal one calculated under the condition without any loss. Through theoretical considerations, it is confirmed that the decrease of velocity due to both the heat loss to the wall and the leakage of charged air through the piston rings is remarkable in the period after 20 deg. before t.d.c. and becomes maximum at top dead center. However, the absolute value of decrement seems to be too small to affect seriously the squish velocity.