To provide a member installation structure for simply and securely installing a member such as an installation metal fitting and an option member to a desired position to a frame or other various long members lengthwise of them.

Using a continuous opening 300 of a horizontal member 32 or 33 as a long member, an extension member 6 is installed to a desired position lengthwise of it. In the horizontal member 32 or 33, an inner surface Sx is formed close to an opening edge of the continuous opening 300 to be wider than its opening width. The extension member 6 is provided with a lock part 62 insertable from the continuous opening 300 in the horizontal member 32 or 33 inward, and rotatable at an insert position to be engaged with the inner surface Sx close to the opening edge of the continuous opening 300, and an operation part 63 to provide the lock part 62 with insert action and engagement action.