An engine comprising: a cylinder; a piston configured to reciprocate in the cylinder; a crankshaft configured to rotate in coordination with the reciprocation of the piston; a power transmission section configured to transmit reciprocating power of the piston to the crankshaft; a first member and a second member configured to constitute the piston or the power transmission section, to cause facing parts of first and second members (112a, 114a) to face each other in a stroke direction of the piston, and to vary the full length of the piston or the power transmission section in the stroke direction according to a distance between these facing parts in the stroke direction; a hydraulic pressure chamber formed between the facing parts of the first and second members (112a, 114a); and a hydraulic pressure adjustment mechanism configured to supply hydraulic oil to the hydraulic pressure chamber or to discharge the hydraulic oil from the hydraulic pressure chamber, and to thereby change the distance between the facing parts of the first and second members (112a, 114a), characterized in that the hydraulic pressure adjustment mechanism comprises a plunger pump that has a pump cylinder into which the hydraulic oil is guided and a plunger which moves in the pump cylinder in the stroke direction and has one end protruding from the pump cylinder, and that supplies the hydraulic oil in the pump cylinder to the hydraulic pressure chamber by pushing the plunger into the pump cylinder, the plunger pump moves in the stroke direction along with the piston and the power transmission section, and the plunger is pushed into the pump cylinder by receiving a reaction force opposite to reciprocating forces of the piston and the power transmission section. a cylinder; a piston configured to reciprocate in the cylinder; a crankshaft configured to rotate in coordination with the reciprocation of the piston; a power transmission section configured to transmit reciprocating power of the piston to the crankshaft; a first member and a second member configured to constitute the piston or the power transmission section, to cause facing parts of first and second members (112a, 114a) to face each other in a stroke direction of the piston, and to vary the full length of the piston or the power transmission section in the stroke direction according to a distance between these facing parts in the stroke direction; a hydraulic pressure chamber formed between the facing parts of the first and second members (112a, 114a); and a hydraulic pressure adjustment mechanism configured to supply hydraulic oil to the hydraulic pressure chamber or to discharge the hydraulic oil from the hydraulic pressure chamber, and to thereby change the distance between the facing parts of the first and second members (112a, 114a), characterized in that the hydraulic pressure adjustment mechanism comprises a plunger pump that has a pump cylinder into which the hydraulic oil is guided and a plunger which moves in the pump cylinder in the stroke direction and has one end protruding from the pump cylinder, and that supplies the hydraulic oil in the pump cylinder to the hydraulic pressure chamber by pushing the plunger into the pump cylinder, the plunger pump moves in the stroke direction along with the piston and the power transmission section, and the plunger is pushed into the pump cylinder by receiving a reaction force opposite to reciprocating forces of the piston and the power transmission section.