To provide a structure of a combustion chamber in an engine, for decreasing friction by partially accumulating compressed air through a process of providing a pressure reducing chamber in a main chamber, and for decreasing fuel consumption by making use of compressed air accumulated in the expansion stroke as work through a process of blowing out it by spring force.

In a structure of a combustion chamber in an engine, a main chamber 1 and a sub chamber 2 communicate with each other by communication holes 13, 31, and a control valve 4 is arranged in the communication hole 31. A pressure reducing chamber 20 in which a sub piston 21 energized by spring force of a spring 22 is provided in the main chamber 1. The sub piston 21 is moved against spring force to partially accumulate compressed air in the pressure reducing chamber 20 into the pressure reducing chamber 20 in the compression stroke, and the sub piston 21 is operated by spring force to blow out the compressed air accumulated in the pressure reducing chamber 20 to the main chamber 1 in the expansion stroke of the piston 15.