A connection confirming method of an optical connector for confirming a butt-connection state of optical fibers (2, 10) that butt and connect to each other in the optical connector, the optical connector comprising a ferrule including a built-in optical fiber in the inside thereof and a connection mechanism positioned on an opposite side to a connection end face of the ferrule to maintain a butt-connection state between the built-in optical fiber and an optical fiber inserted from the outside, the connection mechanism comprising a base, a cover (7, 8) facing the base, and a clamping portion for elastically clamping the ferrule and the cover (7, 8), wherein the cover (7, 8) is opened with respect to the base by inserting an opening and closing member between the ferrule and the cover (7, 8), and the opening and closing member is formed of an optically transparent material, the connection confirming method comprising: performing a connector connection of a test optical fiber to the built-in optical fiber, the test optical fiber for removing cladding mode light, the test optical fiber being a single-mode transmission type, the test optical fiber being made of quartz glass and having a length of 100 to 200 centimeters, wherein a red laser diode light source for confirming the butt-connection and an optical connector for a test being provided at respective ends of the test optical fiber; substantially removing cladding mode light at a butting end of the built-in optical fiber to the inserted optical fiber by entering red light emitted from the red laser diode light source and having a wavelength of 650 nm into the built-in optical fiber through the test optical fiber; when the inserted optical fiber and the built-in optical fiber are butted to each other, detecting red light leaking from its butt-connection part and the vicinity thereof to the outside of the optical connector as leak light from the opening and closing member; and confirming the butt-connection state by comparing a difference of light intensity of leak light generated before and after the built-in optical fiber and an inserted optical fiber are butted to each other. performing a connector connection of a test optical fiber to the built-in optical fiber, the test optical fiber for removing cladding mode light, the test optical fiber being a single-mode transmission type, the test optical fiber being made of quartz glass and having a length of 100 to 200 centimeters, wherein a red laser diode light source for confirming the butt-connection and an optical connector for a test being provided at respective ends of the test optical fiber; substantially removing cladding mode light at a butting end of the built-in optical fiber to the inserted optical fiber by entering red light emitted from the red laser diode light source and having a wavelength of 650 nm into the built-in optical fiber through the test optical fiber; when the inserted optical fiber and the built-in optical fiber are butted to each other, detecting red light leaking from its butt-connection part and the vicinity thereof to the outside of the optical connector as leak light from the opening and closing member; and confirming the butt-connection state by comparing a difference of light intensity of leak light generated before and after the built-in optical fiber and an inserted optical fiber are butted to each other.