PURPOSE: To detect the size of an original by utilizing a detecting element which detects original density by providing a density sensor along the conveyance path of a recording device which has an automatic original feeding function, and providing a reflecting member which is distinguished from original information opposite the density sensor.

CONSTITUTION: The mechanism which moves the density sensor 32 of the recording device body 1 consists of a rack 40, pinion 41, and stepping motor 42 and moves in one direction at right angles to a conveyance direction synchronously with original feeding. A small piece 43 for reference density, e.g. 1.0 optical density is provided along the movement path of the density sensor 32. The density sensor 32' receives intensive reflected light from an original when scanning on the original or weak reflected light from the reference density small piece 43 provided under it in the absence of the original, thereby varying in density. The time when the output level of the sensor 32 is high is counted by the counter in a microcomputer to detect the original size.