PURPOSE: To reliably push out the combustion gas remaining near the inner wall of a cylinder linear and near the center of the cylinder linear at the required timing by providing a hydraulically driven displacer near the center of a piston in a uni-flow scavenging-type two-cycle Diesel engine.

CONSTITUTION: A piston 7 connected to a cank shaft 22 through a crosshead 16 in provided in a uni-flow scavenging-type two-cycle Diesel engine having a scavenging hole 12 and an exhaust hole 27. A scavenging displacer 31 fixed to a rod 32 penetrating the center of the piston is provided on a dish-like piston head section. The rod 32 is controlled by the transfer controller of hydraulic passages 33, 34 feeding oil to hydraulic chambers 53, 54 in the crosshead 16 at the predetermined timing with the crank shaft 22. After the scavenging hole 12 of the piston 17 is opened, the diplacer is moved upward, and the residual gas is reliably discharged at the upper section, while the scavenging air is sucked at the lower section.