An image sensor with a light-receiving-element mounting substrate secured to a frame by bringing the light-receiving-element mounting substrate into close contact with the frame at a uniform and proper pressure, making the optical position of a sensor IC to a manuscript to be read and a rod lens array accurate, and showing a superior resolution. The image sensor comprises a frame which has a first plane contacting with a manuscript to be read and a second plane facing the first plane and in which a hollow portion having an aperture on the first plane and the second plane respectively is formed; a light source for applying light to the manuscript to be read; condensing means for condensing the light reflected from the manuscript to be read; a light-receiving-element mounting substrate set so as to cover the aperture formed on the second plane, a light-receiving element mounted on the mounting plane of the light-receiving-element mounting substrate so as to face the condensing means, and an adhesive tape pasted so as to cover the back of the light-receiving-element mounting substrate and two sides of the frame to secure the light-receiving-element mounting substrate to the second plane by bringing the substrate into close contact with the plane.