A boot liner is configured to adequately obtain a desired sensation of an upper part fitting around the ankle. The boot liner has a sole, an upper part, and a fastening mechanism. The fastening mechanism has a main strap having a first fixed strap part, a pair of movable strap parts, and a second fixed strap part. The movable strap parts extend forwardly from the ends of the first fixed strap part along the side surfaces of the upper part, and are folded back such that an insertion element through which a shoelace can be inserted is formed in the vicinity of the front ends of the upper part, and the distal ends extend through the rear portion to the side surfaces on the opposite sides of the upper part. The movable strap parts intersect each other at the rear portion and can move in relation to the upper part.