To provide a principle and structure of generating motion as a fluid engine which generates differential motion by equally pressurizing mutual cylinders and reciprocates by alternately pressurizing them.

This engine has such structure wherein the thrust of a propulsive cylinder 14, 15 opposes to that of a discharge cylinder 16, 17 by means of the thread take-up lever effect, or directly through the output shaft, and has the system for generating differential thrust in the propulsive cylinders 14, 15 and the discharge cylinders 16, 17 by pressurizing them with low pressure in the anti-propulsive direction opposing to the propulsive direction, to generate differential reciprocating motion. In this engine, pressurizing force of a circulating flow circuit by the circulating flow of the mutual cylinders in equal pressurization is the main input.