A belt length adjuster includes a frame body with two outer frames and two connecting frames, and a support frame provided between the two outer frames and stretching to and between the two connecting frames. The support frame supports one end part of the belt member to be used with the belt length adjuster. Two inner frames are respectively provided between the support frame and the respective outer frames and stretching to and between the two connecting frames. The support frame and the respective inner frames provide first insertion spaces into which the one end part of the belt member is inserted, while the respective inner frames and the respective outer frames provide second insertion spaces into which the other side of the belt member is inserted. Furthermore the one end part of the belt member is easily attached to the frame body.