An attachment unit for an information storage device includes a casing that houses the information storage device and a fixed base on which the casing is detachably mounted. The casing includes a casing body, a pair of rotation supporting shaft portions, a pair of first engagement portions, and a latch portion. The fixed base includes a substantially L-shaped groove, a pair of second engagement portions, and a latch fitting portion. In a state where the casing is mounted on the fixed base, (i) the first engagement portions are in partial contact with the second engagement portions in the first direction, (ii) the casing body is sandwiched in a second direction intersecting the first direction, and (iii) the rotation supporting shaft portions and the latch portion are in partial contact with the grooves and the latch fitting portion, respectively, in a third direction intersecting the first direction and the second direction.