An imager includes: an imaging sensor chip; a fixing member to which the imaging sensor chip is fixed, and which is electrically connected to the imaging sensor chip and has a first linear expansion coefficient; and a circuit board that is fixed to the fixing member via a conductive member, the circuit board comprises a first member having the first linear expansion coefficient and a second member having a second linear expansion coefficient smaller than the first linear expansion coefficient and larger than a linear expansion coefficient of a semiconductor substrate forming the imaging sensor chip, and a content of the second member in the back side portion is greater than a content of the second member in the front side portion.