PURPOSE: To obtain an image being free from non-uniformity in density and color and having a stable quality, even when the temperature of an apparatus fluctuates.

CONSTITUTION: A triangular wave signal is generated by integrating an ECL pixel clock signal from a receiver 202 by an integrating circuit constructed of a resistor 258, a variable resistor 272 and a capacitor 278. Low-frequency fluctuation of the signal at an inverted input end of an OP amplifier 282 is inverted and amplified by an integrating circuit constructed of a resistor 260, a capacitor 276 and an OP amplifier 280, attenuated by resistors 262 and 264 and inputted to a noninverted input end of the OP amplifier 282. Therefore, an input offset amount of the OP amplifier 282 of a reverse phase input decreases and a normal level of the generated triangular wave signal becomes stable. Accordingly, a stable pulse width signal based on comparison of the triangular wave signal with a signal of a level in conformity with image data and corresponding to the image data can be formed.