To provide an in-vehicle electronic apparatus or the like that can be easily installed by a user himself or herself even after a vehicle is purchased, can dispense with wiring to get information from the vehicle, can be installed with reduced cost and labor, and has a convenient fuel mileage display function.

An amount of oil supply is input from a remote controller when oil is supplied at every time. Average fuel mileage displayed on an average fuel mileage display section 54 is the sum of fuel mileage up to now, and therefore is differ from a section-average fuel mileage displayed on a section-average fuel mileage display section 53. Then the section-average fuel mileage is compared with the average fuel mileage. If the section-average fuel mileage and the average fuel mileage are in agreement, characters in the section-average fuel mileage display section 53 are displayed in black. If the section-average fuel mileage exceeds the average fuel mileage, the characters in the section-average fuel mileage display section 53 are displayed in blue. If the section-average fuel mileage is less than the average fuel mileage, the characters in the section-average fuel mileage display section 53 are displayed in red.