Image forming apparatus

An image forming apparatus, comprising:

• an image bearing member;
• an image forming unit configured to form an image on the image bearing member;
• a transfer unit configured to transfer the image from the image bearing member to a sheet;
• 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,
• wherein 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 of the circuit board, which is opposite to the first side,
• wherein the attachment member has a surface contacting the second side of the circuit board, and
• wherein 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.