An image sensor unit includes: sensor substrates on which a plurality of sensor chips are mounted; a plurality of rod-lens arrays that focus light from an original on the sensor substrates; and a frame body that houses the plurality of sensor substrates and the plurality of rod-lens arrays. The frame body is divided into a first frame and a second frame. A side surface of the plurality of rod-lens arrays in a sub-scan direction is fixed only by the first frame, and the plurality of rod-lens arrays are arranged in the main-scan direction. The plurality of rod-lens arrays is fixed to the first frame by applying a first adhesive and a second adhesive, respectively, the viscosity of the first adhesive exceeds the viscosity of the second adhesive, and the adhesive range of the first adhesive is greater than the adhesive range of the second adhesive.