To provide a troubleshooting device of a fuel level sensor capable of rapidly detecting troubles by increasing the frequencies of the determination of troubles in the fuel level sensor.

When the absolute value |VP-VPFLSTZ| of the varied amount of a vehicle speed VP is equal to or higher than a vehicle speed variation determination value DVPFLST (S71), if the absolute value |FLFLM-FLVLSTCO| of the varied amount of a fuel level FLFLM detected by a fuel level sensor 11 is equal to or higher than a prescribed fuel level varied amount FLVLSTCWD, an OK counter COKFLST is incremented (S73 to S75). When the ratio of the value of the OK counter COKFLST to the determination execution frequencies CTFLFTDV is smaller than a determination ratio FLSTJUD, the fuel level sensor 11 is determined to be defective (S76, S79).