A two-stroke engine comprising: a cylinder block formed with a cylinder and a crank chamber; and a piston slidably mounted in the cylinder, wherein the cylinder block includes: an exhaust passage leading to a combustion chamber in the cylinder through an exhaust port opened to an inner circumferential surface of the cylinder; a scavenging port opened to the inner circumferential surface of the cylinder; a communication passage extending from the scavenging port in a radial direction of the cylinder; and a scavenging passage extending in an axial direction of the cylinder, communicating with the crank chamber, and having an opening portion formed in a bottom surface of the communication passage, wherein an inner surface of the scavenging passage on a cylinder side comprises a protrusion at a position on a communication passage side, the protrusion of the inner surface of the scavenging passage protruding outward in a radial direction of the cylinder, wherein a ceiling surface of the communication passage is inclined toward a cylinder head with increasing distance from a scavenging passage side thereof toward the scavenging port, and a bottom surface of the communication passage is inclined toward the crank chamber with increasing distance from a scavenging passage side thereof toward the scavenging port. a cylinder block formed with a cylinder and a crank chamber; and a piston slidably mounted in the cylinder, wherein the cylinder block includes: an exhaust passage leading to a combustion chamber in the cylinder through an exhaust port opened to an inner circumferential surface of the cylinder; a scavenging port opened to the inner circumferential surface of the cylinder; a communication passage extending from the scavenging port in a radial direction of the cylinder; and a scavenging passage extending in an axial direction of the cylinder, communicating with the crank chamber, and having an opening portion formed in a bottom surface of the communication passage, wherein an inner surface of the scavenging passage on a cylinder side comprises a protrusion at a position on a communication passage side, the protrusion of the inner surface of the scavenging passage protruding outward in a radial direction of the cylinder, wherein an exhaust passage leading to a combustion chamber in the cylinder through an exhaust port opened to an inner circumferential surface of the cylinder; a scavenging port opened to the inner circumferential surface of the cylinder; a communication passage extending from the scavenging port in a radial direction of the cylinder; and a scavenging passage extending in an axial direction of the cylinder, communicating with the crank chamber, and having an opening portion formed in a bottom surface of the communication passage, wherein an inner surface of the scavenging passage on a cylinder side comprises a protrusion at a position on a communication passage side, the protrusion of the inner surface of the scavenging passage protruding outward in a radial direction of the cylinder, wherein a ceiling surface of the communication passage is inclined toward a cylinder head with increasing distance from a scavenging passage side thereof toward the scavenging port, and a bottom surface of the communication passage is inclined toward the crank chamber with increasing distance from a scavenging passage side thereof toward the scavenging port.