To provide a hydraulic rotary engine which is free from environmental pollution and warming, absence in noise and vibration, requires no fuel, can be constructed and operated in cities, mountains, and local districts, and profitable even if it takes its resources from a sea bottom, sea water, or fields and mountains, purifies sea water, and feeds it to desert.

When an automatic rotating adjusting motor 29 rotates a main gear 25, a buoyancy piston 5 rises in liquid 31 contained in an outer case 10 so as to form the association of a fixed gear 12, connecting gear 8, and clutch 33 in continuous rotation. In addition, the structural size, shape, number of cylinders, and number of tubes are not limited.