Image reading apparatus and image forming apparatus

An image reading apparatus includes: a light amount adjustment portion which performs light amount adjustment on a light source such that the maximum value of the analog output of an image sensor does not exceed the upper limit value of an input voltage range of an A/D conversion portion but approaches the upper limit value; a temperature sensor which measures a temperature; and a storage portion which stores a temperature at the time of the light amount adjustment. When an absolute value of a temperature difference between a temperature stored in the storage portion and the current temperature is less than a threshold value, the light amount adjustment portion does not perform the light amount adjustment whereas when the absolute value is equal to or more than the threshold value, the light amount adjustment portion performs the light amount adjustment.