An image sensor of the kind which employs a line array of a-Si photodiodes on an insulating substrate and in which the outputs of the diodes are stored in the lead capacitances until readout. To increase the size of the lead capacitance and thereby to improve the linearity of the ouput, the lead between each diode and its processing circuit is made one plate of a capacitor, of which the other plate is a metal film insulated from the lead and advantageously maintained at ground.