An image sensor unit includes: a plurality of sensor substrates that are connected in a main-scan direction and that are provided with sensor chips in the main-scan direction, the sensor chips converting light from an original to electric signals; and a plurality of rod-lens arrays that are connected in the main-scan direction and that are provided with a plurality of rod lenses in the main-scan direction, the rod lenses focusing the light from the original on the sensor chips, wherein connection positions between the plurality of sensor substrates are arranged at positions not overlapping with connection positions between the plurality of rod-lens arrays. A decrease in reading accuracy of an image can be reduced even if short constituent members are connected in the main-scan direction to form an elongated image sensor unit.