1. A facsimile transmitter, comprising: image-taking means for scanning image information in an original along a principal scanning direction to convert said information into electric signals; original displacing means for displacing said original in an auxiliary scanning direction substantially perpendicular to said principal scanning direction; image-forming means for focusing an optical image with one of plural image sizes on said image-taking means in order to obtain image signals for an equal-size image or a modified-size image of the information on said original from said image-taking means; image size detecting means for detecting information corresponding to the image size of the optical image focused on said image-taking means upon detection of whether said image-forming means is in one position or is in another position; and control means for controlling the amount of displacement of said original by said original displacing means in response to the detection by said size detecting means.