A linear image sensor comprises linear image sensor ICs each having light receiving elements arranged according to a preselected resolution level for outputting output signals in accordance with an amount of light received. Switch elements interconnect output terminals of adjacent two or more of the light receiving elements for switching between the preselected resolution level of the light receiving elements to at least one other resolution level constituting a fraction of the preselected resolution level. First amplifier circuits are is connected to output terminals of the light receiving elements. Sample and hold circuits temporarily hold outputs of the first amplifier circuits. Second amplifier circuits are connected to output terminals of the sample and hold circuits. Reading switch elements read outputs of the second amplifier circuits. Scanning circuits control the reading switch elements.