Image forming apparatus with suppression of light quality attenuation

An image forming apparatus includes a container to accommodate a developer, and a sensor to output an output signal depending on an amount of the developer in the container, wherein the sensor includes a light emitting element and a light receiving element which are provided outside the container, and a light guide provided on a wall surface of the container and configured to guide light, emitted by the light emitting element, toward the light receiving element through an inside space of the container. The light guide also includes projected portions configured such that light travels from outside of the container to the inside of the container in a direction intersecting with the direction of gravity.