Stereoscopic photography using four mirrors and a 16 mm high speed camera has been developed to observe in-cylinder gas motion three dimensionally. The one-camera system can simultaneously produce both left and right pictures under the same optical conditions. The measurable range of the system is from 400 to 500 mm and the resolution is 1.7mm. In-cylinder phenomena can be observed cycle-by-cycle without any modification of a cylinder head.To confirm its effectiveness, the system was applied to a bottom view type single-cylinder S.I engine with four valves. The bore and stroke are 92mm and 75mm, respectively. The rotation speed in a test was 1500rpm and a volume efficiency was 40%.The film speed of the high speed camera ranged from 3,000 to 6,000 pictures per second. In-cylinder gas motion during combustion was examined three-dimensionally by tracking bright flame spots produced by a sodium compound added to the fuel. Two typical gas motions, swirl and tumbling were generated by changing shape of intake ports. In the case of swirl motion, a simple vortex was observed and vertical flows were seen frequently on the intake valve side. In case of tumbling motion, two contraction flows directed against each other and parallel to the ridge of the pent-roof of the head together with a flow directed from the intake to the exhaust valve were observed.