To provide an adjuster for a fuel gage to sufficiently reduce discrepancy between an actual fuel level and a detection value of a fuel gage even if manufacture irregularity is found in a fuel tank.

With the fuel tank mounted with the fuel gage kept in a still standing state, this adjuster reads in an output of the float-type fuel gage while reading in an output of the fuel gage with the fuel tank kept in an inverted state. The adjuster learns corrective values used for causing output characteristics shown by the read-in outputs to coincide with previously set output characteristics.