Patent ID: 8320814

Claim:
A length measurement apparatus comprising: a length measurement roll that comes in contact with a sheet conveyed on a conveying path, and rotates along with the conveyance of the sheet; a first sensor that is disposed on an upstream side of the length measurement roll in a sheet conveying direction, and detects the sheet conveyed on the conveying path; a second sensor that is disposed on the upstream side or a downstream side of the length measurement roll in the sheet conveying direction, and detects the sheet conveyed on the conveying path; a third sensor that that is disposed on the downstream side of the length measurement roll in the sheet conveying direction, and detects the sheet conveyed on the conveying path; a measurement portion that measures a first sheet length of the sheet based on a rotational amount of the length measurement roll for a first detection period in which the first and third sensors detect the sheet, and measures a second sheet length of the sheet 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 in the sheet conveying direction; 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 first and second sheet lengths, and calculates the whole length of the sheet in the sheet conveying direction by using the selected sheet length.