A length measurement apparatus including: a measurement portion that measures a sheet length based on a rotational amount of a length measurement roll for a first detection period in which first and third sensors detect the sheet, and a sheet length based on a rotational amount of the length measurement roll for a second detection period in which the second sensor and any one of the first and third sensors detect the sheet, the any one of the first and third sensors being disposed at a position opposite to the second sensor via the length measurement roll; and a whole length calculation portion that selects the sheet length nearer to integral multiples of the circumference length of the length measurement roll from the sheet lengths measured for the first and second detection periods, and calculates the whole length of the sheet by using the selected sheet length.