An image reading device comprising: a photoelectric conversion device configured to receive light reflected from an original and convert received light to an image signal; a spectrum spread clock generator circuit, SSCG, configured to output a frequency-modulated clock a timing generator configured to generate drive clocks for the photoelectric conversion device and for a signal combining unit based on the frequency-modulated clock; a modulation signal detector configured to detect and output a modulation signal being a signal corresponding to a change in frequency of the frequency-modulated clock; an amplifier configured to invert the phase of the modulation signal, thereby obtaining a phase-inverted modulation signal; an amplitude controller configured to control an amplitude of the phase-inverted modulation signal outputted from the modulation signal detector thereby generating an amplitude-controlled modulation signal and to output the amplitude-controlled modulation signal, wherein the control procedure comprises a process of detecting a variation occurring in an image signal due to frequency modulation, and a process of increasing or decreasing the amplitude of the phase-inverted modulation signal such that an amplitude of the phase-inverted modulation signal is caused to coincide with the detected variation; the signal combining unit configured to superimpose the amplitude-controlled modulation signal on the image signal in order to suppress disturbance due to the SSCG; a buffer, provided in a stage subsequent to the amplitude controller, configured to buffer the amplitude-controlled modulation signal outputted from the amplitude controller; and a decoupling capacitor, provided between the buffer and the signal combining unit; wherein the signal combining unit is adapted to superimpose the amplitude-controlled modulation signal buffered in the buffer unit on the image signal; wherein detecting the variation comprises acquiring black level data, detecting a maximum value and a minimum value of the black level data, and detecting a difference between the maximum value and the minimum value as the variation; wherein the output from the buffer of the modulation signal is controlled with a control signal (SS_CTL) such that the modulation signal is provided to the signal combining unit only when the variation of the image signal due to the SSCG occurs. a photoelectric conversion device configured to receive light reflected from an original and convert received light to an image signal; a spectrum spread clock generator circuit, SSCG, configured to output a frequency-modulated clock a timing generator configured to generate drive clocks for the photoelectric conversion device and for a signal combining unit based on the frequency-modulated clock; a modulation signal detector configured to detect and output a modulation signal being a signal corresponding to a change in frequency of the frequency-modulated clock; an amplifier configured to invert the phase of the modulation signal, thereby obtaining a phase-inverted modulation signal; an amplitude controller configured to control an amplitude of the phase-inverted modulation signal outputted from the modulation signal detector thereby generating an amplitude-controlled modulation signal and to output the amplitude-controlled modulation signal, wherein the control procedure comprises a process of detecting a variation occurring in an image signal due to frequency modulation, and a process of increasing or decreasing the amplitude of the phase-inverted modulation signal such that an amplitude of the phase-inverted modulation signal is caused to coincide with the detected variation; the signal combining unit configured to superimpose the amplitude-controlled modulation signal on the image signal in order to suppress disturbance due to the SSCG; a buffer, provided in a stage subsequent to the amplitude controller, configured to buffer the amplitude-controlled modulation signal outputted from the amplitude controller; and a decoupling capacitor, provided between the buffer and the signal combining unit; wherein the signal combining unit is adapted to superimpose the amplitude-controlled modulation signal buffered in the buffer unit on the image signal; wherein detecting the variation comprises acquiring black level data, detecting a maximum value and a minimum value of the black level data, and detecting a difference between the maximum value and the minimum value as the variation; wherein the output from the buffer of the modulation signal is controlled with a control signal (SS_CTL) such that the modulation signal is provided to the signal combining unit only when the variation of the image signal due to the SSCG occurs.