In a premixed compression self-ignition engine which is compact but capable of exerting a high output, an intermediate portion of a trigonal link is rotatably supported on a crankpin, a large end portion of a connecting rod is connected to one end portion of the trigonal link, and a swing link swingably supported on a fulcrum shaft is connected to another end portion of the trigonal link. The engine includes: an intake port open to a crank chamber; a reed valve mounted in the intake port; a scavenging port configured to guide fresh air from the crank chamber into a cylinder bore in a downward stroke of a piston; an exhaust port provided in a cylinder head and open to a top portion of the cylinder bore; and an exhaust valve configured to be opened before the piston opens the scavenging port in the downward stroke of the piston.