A variable length connecting rod assembly comprising a variable length connecting rod (6, 6a, 6b, 6c, 6d, 6e); wherein the variable length connecting rod (6, 6a, 6b, 6c, 6d, 6e) comprises: a connecting rod body provided with a crank receiving opening for receiving a crank pin; an eccentric member provided with a piston pin receiving opening for receiving a piston pin and attached to the connecting rod body to be able to turn so as to make a length between a center of the piston pin receiving opening and a center of the crank receiving opening change; a hydraulic cylinder formed at the connecting rod body and supplied with hydraulic oil; a hydraulic piston configured to slide inside the hydraulic cylinder and to move in linkage with the eccentric member; and a flow direction switching mechanism provided at the connecting rod body; the variable length connecting rod assembly further comprises a hydraulic mechanism (34a, 50, 38) handling the hydraulic oil separate from the hydraulic cylinder, the hydraulic piston, and the flow direction switching mechanism; the flow direction switching mechanism comprises a single check valve and a single switching mechanism, the check valve is configured to permit a flow of hydraulic oil from a primary side of the check valve to a secondary side of the check valve and prohibit the flow of hydraulic oil from the secondary side to the primary side, the switching mechanism comprises a single switching pin, the switching pin is held in a pin holding space formed in the connecting rod body, the connecting rod body is formed with a first oil path (51, 52, 60) making the hydraulic cylinder and the pin holding space communicate, a second oil path (53, 54, 59, 61, 62, 62a, 62b, 64, 65) making the hydraulic mechanism (34a, 50, 38) or the hydraulic mechanism (34a, 50, 38) and an outside space of the variable length connecting rod (6, 6a, 6b, 6c, 6d, 6e), and the pin holding space communicate, a third oil path (56, 58) making the primary side of the check valve and the pin holding space communicate, and a fourth oil path (55, 57) making the secondary side of the check valve and the pin holding space communicate, and the switching pin is configured to be switched between a first position making the first oil path (51, 52, 60) communicate with the fourth oil path (55, 57) and making the second oil path (53, 54, 59, 61, 62, 62a, 62b, 64, 65) communicate with the third oil path (56, 58), and a second position making the first oil path (51, 52, 60) communicate with the third oil path (56, 58) and making the second oil path (53, 54, 59, 61, 62, 62a, 62b, 64, 65) communicate with the fourth oil path (55, 57). a connecting rod body provided with a crank receiving opening for receiving a crank pin; an eccentric member provided with a piston pin receiving opening for receiving a piston pin and attached to the connecting rod body to be able to turn so as to make a length between a center of the piston pin receiving opening and a center of the crank receiving opening change; a hydraulic cylinder formed at the connecting rod body and supplied with hydraulic oil; a hydraulic piston configured to slide inside the hydraulic cylinder and to move in linkage with the eccentric member; and a flow direction switching mechanism provided at the connecting rod body;