The present invention provides an image sensor unit, an image reading apparatus, and an image forming apparatus. In the image sensor unit, a frame includes a light blocking portion that protrudes toward one surface of a circuit board at a position between a light source and an image sensor. On the one surface of the circuit board, a conductive portion that continuously overlaps with external lines of the light blocking portion in a sub-scan direction in a view of the circuit board in a direction orthogonal to the one surface is formed.