An image forming apparatus includes an image bearing member, an optical sensor configured to detect a pattern image formed on the image bearing member, and an attachment member to which the optical sensor is attached. The optical sensor includes a circuit board, a light emitting element and a light receiving element which are provided on a first side of the circuit board, and a connector provided on a second side opposite to the first side of the circuit board. The attachment member has a surface contacting the second side of the circuit board. In an insertion/removal direction of a cable to be connected to the connector, a first thickness, from the surface, of a part of a portion, contacting the second side, of the attachment member is smaller than a second thickness, from the surface, of a portion, not contacting the second side, of the attachment member.