Disclosed is a tool for releasing an engaged state in which a receptacle-side engagement section formed on a side surface of a receptacle provided on a substrate is engaged with a connector-side engagement section of an optical connector that is attachable to and detachable from the receptacle. The tool includes: release claws to be inserted into respective gaps, each gap being formed between a member arranged between a pair of engagement side plate parts each being provided with the connector-side engagement section, and each engagement side plate part; and a guide part that comes into opposition with a side surface of the optical connector or the receptacle. By bringing the guide part into opposition with the side surface and sliding the release claws toward the substrate, the engagement side plate parts are spread outward by the release claws and the engaged state is released.