To permit the acquisition of all data indicating the real operating conditions of an internal-combustion engine visually or by high-speed photographing, by providing a constitution in which the side thrust of a piston can be reduced and forming a cylinder out of transparent glass, etc.

A glass cylinder 11 formed of transparent glass is clamped between clamp plates 5a and 5b by bolts 6a and 6b via cylinder heads 12a and 12b, and a double headed piston 16 is inserted into the cylinder 11. In addition, inlet valves 13a and 13b, and exhaust valves 14a and 14b, and a spark plug connected to output shafts 28a and 28b via a coupling mechanism are installed to the cylinder heads 12a and 12b. As it is possible to directly see the operating conditions of all the elements concerning combustion such as the piston 16, inlet valves 13a and 13b, exhaust valves 14a and 14b, spark plug, piston rings, state of lubrication in this constitution, it is possible to acquire information related to the combustion of an engine with high accuracy.