An adapter used to connect an optical cable to a remotely operated vehicle, ROV, used underwater, the adapter comprising: a photoelectric conversion device; and a first attachment structure for detachably attaching the adapter to the ROV in cooperation with a second attachment structure of the ROV, wherein: the photoelectric conversion device comprises a waterproof structure, a first connector used to connect the optical cable to the photoelectric conversion device, and a second connector used to connect the photoelectric conversion device to the ROV via a communication cable; and the second connector is a second communication connector configured to be detachably attached to a first communication connector positioned at one end of a communication cable extending from the ROV; and the first communication connector and the second communication connector are respectively male and female connectors, and a contact area of end surface of pins of the first communication connector and the second communication connector is larger than a contact area of the first connector and the optical cable, wherein the first attachment structure is configured to attach to the second attachment structure in at least one of the following methods without use of tools: a method in which a lock hook is used; a method in which a latch lock is used; a method in which a fastener is used; and a method in which a magnet is used. a photoelectric conversion device; and a first attachment structure for detachably attaching the adapter to the ROV in cooperation with a second attachment structure of the ROV, wherein: the photoelectric conversion device comprises a waterproof structure, a first connector used to connect the optical cable to the photoelectric conversion device, and a second connector used to connect the photoelectric conversion device to the ROV via a communication cable; and the second connector is a second communication connector configured to be detachably attached to a first communication connector positioned at one end of a communication cable extending from the ROV; and the first communication connector and the second communication connector are respectively male and female connectors, and a contact area of end surface of pins of the first communication connector and the second communication connector is larger than a contact area of the first connector and the optical cable, wherein the first attachment structure is configured to attach to the second attachment structure in at least one of the following methods without use of tools: a method in which a lock hook is used; a method in which a latch lock is used; a method in which a fastener is used; and a method in which a magnet is used. the photoelectric conversion device comprises a waterproof structure, a first connector used to connect the optical cable to the photoelectric conversion device, and a second connector used to connect the photoelectric conversion device to the ROV via a communication cable; and the second connector is a second communication connector configured to be detachably attached to a first communication connector positioned at one end of a communication cable extending from the ROV; and the first communication connector and the second communication connector are respectively male and female connectors, and a contact area of end surface of pins of the first communication connector and the second communication connector is larger than a contact area of the first connector and the optical cable, wherein the first attachment structure is configured to attach to the second attachment structure in at least one of the following methods without use of tools: a method in which a lock hook is used; a method in which a latch lock is used; a method in which a fastener is used; and a method in which a magnet is used. a method in which a lock hook is used; a method in which a latch lock is used; a method in which a fastener is used; and a method in which a magnet is used.