A plurality of opto-electro conversion elements are arranged integrally in a row to constitute a line image sensor to produce respective image bit signals. A plurality of switching elements are connected to the corresponding conversion elements and switchable between on and off states to read out the corresponding bit image signals. A scanning circuit scans sequentially the switching elements to successively turn on them to thereby effect the sequential reading of bit image signals. An initiating circuit is connected to the scanning circuit for generating an initiating signal effective to initiate the scanning circuit. An ending circuit is also connected to the scanning circuit and operates after the switching element of last stage is turned off for generating an ending signal indicative of the ending of scanning operation for one line. A control circuit receives the initiating and ending signals and produces a control signal. A gate circuit is commonly connected to the switching elements and made open according to the control signal to output the series of image bit signals.