Sensor mounting structure for engine

The structure has a fastener (24) for mounting a sensor (25) on an outer wall (11) of a crankcase (2) below an axis (3) of a crankshaft for detecting engines driving conditions. A cover unit (29) covers the sensor and extends from an outer wall of the crankshaft in a direction along an outer surface of the crankcase. The cover unit is provided between exhaust pipes and the sensor.