A loop scavenged two-stroke engine provided with a pair of left and right scavenging passages (31L, 31R) provided symmetrically with respect to a center line of an opening of an exhaust port which adopt a reverse scavenging system, wherein - a combustion operation chamber formed above a piston and a crank-chamber into which the air-fuel mixture is taken communicate with each other via the scavenging passages (31L, 31R), - a scavenging angle (θa or θb) - as seen in cross-sectional view of the two-stroke engine, the scavenging angle (θa,θb) being an angle of intersection of an extension line (Ea) of a guide wall surface of an intake port side of a scavenging passage (31L, 31R) and the exhaust port center line - in at least one spot of a first scavenging passage of the pair of left and right scavenging passages (31L, 31R) located at one side is made to differ from a respective scavenging angle (θa or θb) of a second scavenging passage of the pair of left and right scavenging passages (31L, 31R) located at the other side, and - scavenging flows are sucked into the combustion operating chamber through the scavenging passages (31L, 31R), and - a main flow of a scavenging flow which is blown out from the first scavenging passage located at the one side and a main flow of a scavenging flow of the air-fuel mixture which is blown out from the second scavenging passage which is located at the other side collide with each other in plane view in a region displaced in a lateral direction from the center line of an opening of the exhaust port, characterized in that - a cross-sectional shape of the first scavenging passage located at the one side is formed into a shape of a triangle with rounded corners, and a cross-sectional shape of the second scavenging passage located at the other side is formed into a shape of a parallelogram with rounded corners. - a combustion operation chamber formed above a piston and a crank-chamber into which the air-fuel mixture is taken communicate with each other via the scavenging passages (31L, 31R), - a scavenging angle (θa or θb) - as seen in cross-sectional view of the two-stroke engine, the scavenging angle (θa,θb) being an angle of intersection of an extension line (Ea) of a guide wall surface of an intake port side of a scavenging passage (31L, 31R) and the exhaust port center line - in at least one spot of a first scavenging passage of the pair of left and right scavenging passages (31L, 31R) located at one side is made to differ from a respective scavenging angle (θa or θb) of a second scavenging passage of the pair of left and right scavenging passages (31L, 31R) located at the other side, and - scavenging flows are sucked into the combustion operating chamber through the scavenging passages (31L, 31R), and - a main flow of a scavenging flow which is blown out from the first scavenging passage located at the one side and a main flow of a scavenging flow of the air-fuel mixture which is blown out from the second scavenging passage which is located at the other side collide with each other in plane view in a region displaced in a lateral direction from the center line of an opening of the exhaust port, - a cross-sectional shape of the first scavenging passage located at the one side is formed into a shape of a triangle with rounded corners, and a cross-sectional shape of the second scavenging passage located at the other side is formed into a shape of a parallelogram with rounded corners.