An image sensor (100) comprises a lens (4), a sensor (6) and a first casing (1). The lens (4) is configured to focus light irradiated toward an object to be read (30) from a direction tilted relative to the X-Z plane, and reflected by the object to be read (30). The sensor (6) is configured to receive the light focused by the lens (4). The first casing (1) is configured to contain or retain the lens (4) and the sensor (6) and to have, in a surface (1j) extending along a main scanning direction, a tilted portion having a length in a sub-scanning direction that decreases toward the object to be read (30).