A new type of rapid compression and expansion machine (RCEM) has been developed, and typical knock scenes were clearly recorded with a high speed laser shadowgraph at a speed of 100,000 frames per second.The RCEM is intended to simulate combustion in an automotive engine. Its piston is driven by an electrohydraulic servo system and is allowed to execute continuous reciprocations up to five times. The combustion chamber is a simple pancake type with an ignition plug on its side and the whole inner view is observable through a glass window on the top.Knock observation was made under the following conditions; (1) the fuel was butane, (2) the charged gas was homogeneously pre-mixed and static, (3) the piston executed a single reciprocation. Other parameters were set for heavy knock to occur.The shadowgraph observation revealed that autoignition occurs at a point in the endgas far from a normal flame front and is reflected by the opposite wall. Comparison with dynamic pressure measurements suggests that the wave is a shock wave, which causes an abrupt pressure rise followed by vibration. Knock can be explained by the autoignition and the subsequent pressure wave propagation process.