An image signal processing circuit for use in an image reading device having a scanner controller which generates a gate control signal (SHGT), the image signal processing circuit comprising: inputting means (11, 12) for inputting and clamping an image signal, obtained by optically reading image information contained in an original under the control of the scanner controller; sampling means arranged to sample and hold a specified range of the image signal to output a sampled image signal; analog-to-digital converter means arranged to convert the sampled image signal, before or after amplification, from analog-to-digital form; amplifying means arranged to amplify the sampled image signal, or image data converted from the sampled image signal, to digital form, with a gain to output amplified image data; automatic gain control means arranged to control the gain of the amplifying means by detecting a signal level of the amplified and analog-to-digital converted sampled image signal, to determine whether the detected signal level is equal to a white target level, stored in a memory in the automatic gain control means, to generate a determination result, to obtain an adjusted value of the gain of the amplifying means based on the determination result, to set the gain of the amplifying means to the adjusted value such that the signal level of the amplified and analog-to-digital converter sampled signal is closer to the target level, and then to cease detecting the signal level of the specified range of the image signal; and timing generation and interface means connectable to the scanner controller to supply timing and control signals (SCLK, SD, CS, SHGT) from the scanner controller to the inputting means, sampling means, analog-to-digital converter means and automatic gain control means, the timing generation and interface means being responsive to the gate control signal (SHGT) to generate line synchronization signals (XLSYNC) which are supplied to the automatic gain control means, and the automatic gain control means being arranged to start detecting the signal level in response to either: the gate control signal (SHGT) input to the automatic gain control means from the timing generation and interface means in synchronization with the line synchronization signal (XLSYNC); or a register control signal (REG_Ctrl) input to the automatic gain control means from the timing generation and interface means in synchronization with the line synchronization signal. inputting means (11, 12) for inputting and clamping an image signal, obtained by optically reading image information contained in an original under the control of the scanner controller; sampling means arranged to sample and hold a specified range of the image signal to output a sampled image signal; analog-to-digital converter means arranged to convert the sampled image signal, before or after amplification, from analog-to-digital form; amplifying means arranged to amplify the sampled image signal, or image data converted from the sampled image signal, to digital form, with a gain to output amplified image data; automatic gain control means arranged to control the gain of the amplifying means by detecting a signal level of the amplified and analog-to-digital converted sampled image signal, to determine whether the detected signal level is equal to a white target level, stored in a memory in the automatic gain control means, to generate a determination result, to obtain an adjusted value of the gain of the amplifying means based on the determination result, to set the gain of the amplifying means to the adjusted value such that the signal level of the amplified and analog-to-digital converter sampled signal is closer to the target level, and then to cease detecting the signal level of the specified range of the image signal; and timing generation and interface means connectable to the scanner controller to supply timing and control signals (SCLK, SD, CS, SHGT) from the scanner controller to the inputting means, sampling means, analog-to-digital converter means and automatic gain control means, the timing generation and interface means being responsive to the gate control signal (SHGT) to generate line synchronization signals (XLSYNC) which are supplied to the automatic gain control means, and the automatic gain control means being arranged to start detecting the signal level in response to either: the gate control signal (SHGT) input to the automatic gain control means from the timing generation and interface means in synchronization with the line synchronization signal (XLSYNC); or a register control signal (REG_Ctrl) input to the automatic gain control means from the timing generation and interface means in synchronization with the line synchronization signal. the gate control signal (SHGT) input to the automatic gain control means from the timing generation and interface means in synchronization with the line synchronization signal (XLSYNC); or a register control signal (REG_Ctrl) input to the automatic gain control means from the timing generation and interface means in synchronization with the line synchronization signal.