A straddled vehicle comprising: an engine unit including: a crankshaft; a crankshaft supporter supporting the crankshaft; a gearbox having gear positions with which a rotation speed of the crankshaft is changed at different transmission ratios, respectively, and power input from the crankshaft is transmitted to a driving wheel and a gear position in which the power input from the crankshaft is not transmitted to the driving wheel; and a gearbox supporter supporting the gearbox, the crankshaft supporter and the gearbox supporter being integrally molded; a contactless engine rotation speed sensor configured to detect an engine rotation speed; a contactless vehicle speed sensor configured to detect a vehicle speed; a gear position sensor provided in the gearbox supporter to detect the gear position of the gearbox; and a gear position sensor failure detector configured to detect a failure of the gear position sensor, the gear position sensor failure detector including: a gear position estimation unit configured to estimate the gear position of the gearbox based on a signal from the engine rotation speed sensor and a signal from the vehicle speed sensor; and a failure determination unit configured to determine whether there is a failure of the gear position sensor by comparing the gear position estimated by the gear position estimation unit with the gear position detected by the gear position sensor. an engine unit including: a crankshaft; a crankshaft supporter supporting the crankshaft; a gearbox having gear positions with which a rotation speed of the crankshaft is changed at different transmission ratios, respectively, and power input from the crankshaft is transmitted to a driving wheel and a gear position in which the power input from the crankshaft is not transmitted to the driving wheel; and a gearbox supporter supporting the gearbox, the crankshaft supporter and the gearbox supporter being integrally molded; a contactless engine rotation speed sensor configured to detect an engine rotation speed; a contactless vehicle speed sensor configured to detect a vehicle speed; a gear position sensor provided in the gearbox supporter to detect the gear position of the gearbox; and a gear position sensor failure detector configured to detect a failure of the gear position sensor, the gear position sensor failure detector including: a gear position estimation unit configured to estimate the gear position of the gearbox based on a signal from the engine rotation speed sensor and a signal from the vehicle speed sensor; and a failure determination unit configured to determine whether there is a failure of the gear position sensor by comparing the gear position estimated by the gear position estimation unit with the gear position detected by the gear position sensor. a gear position estimation unit configured to estimate the gear position of the gearbox based on a signal from the engine rotation speed sensor and a signal from the vehicle speed sensor; and a failure determination unit configured to determine whether there is a failure of the gear position sensor by comparing the gear position estimated by the gear position estimation unit with the gear position detected by the gear position sensor.