A single-cylinder or in-line multi-cylinder engine is provided. The engine includes a crankshaft, a piston, a cylinder body, a cylinder head, and a gear change mechanism. The piston is connected to the crankshaft. A cylinder is formed in the cylinder body. The piston is housed in the cylinder. The cylinder head is attached to an end of the cylinder body. The gear change mechanism is disposed to the rear of the crankshaft. An axis of the cylinder extends upward obliquely to the rear from the crankshaft side. The axis of the cylinder passes in front of an axis of the crankshaft when viewed from a direction in which the crankshaft extends.