A liquid supply unit mountable to a liquid ejection device, comprising: a bottom wall located on a bottom when the liquid supply unit is mounted to the liquid ejection device; an upper wall opposed to the bottom wall; a first side wall arranged to intersect with the bottom wall and the upper wall; a second side wall arranged to intersect with the bottom wall and the upper wall and opposed to the first side wall; a first end wall arranged to intersect with the bottom wall, the upper wall, the first side wall and the second side wall; a second end wall arranged to intersect with the bottom wall, the upper wall, the first side wall and the second side wall and opposed to the first end wall; and a liquid retaining member placed in a space surrounded by the bottom wall, the upper wall, the first side wall, the second side wall, the first end wall and the second end wall, characterized in that the first side wall includes a first part and a second part aligned in a first direction from the first end wall toward the second end wall, the first part of the first side wall has an outer surface comprising: a first area (425a1) located closer to the bottom wall than the upper wall and arranged to be substantially perpendicular to the bottom wall; and a second area (425a2) located closer to the upper wall than the bottom wall and arranged to be inclined with respect to the bottom wall, and the second part of the first side wall is arranged to be inclined with respect to the bottom wall. a bottom wall located on a bottom when the liquid supply unit is mounted to the liquid ejection device; an upper wall opposed to the bottom wall; a first side wall arranged to intersect with the bottom wall and the upper wall; a second side wall arranged to intersect with the bottom wall and the upper wall and opposed to the first side wall; a first end wall arranged to intersect with the bottom wall, the upper wall, the first side wall and the second side wall; a second end wall arranged to intersect with the bottom wall, the upper wall, the first side wall and the second side wall and opposed to the first end wall; and a liquid retaining member placed in a space surrounded by the bottom wall, the upper wall, the first side wall, the second side wall, the first end wall and the second end wall, characterized in that the first side wall includes a first part and a second part aligned in a first direction from the first end wall toward the second end wall, the first part of the first side wall has an outer surface comprising: a first area (425a1) located closer to the bottom wall than the upper wall and arranged to be substantially perpendicular to the bottom wall; and a second area (425a2) located closer to the upper wall than the bottom wall and arranged to be inclined with respect to the bottom wall, and a first area (425a1) located closer to the bottom wall than the upper wall and arranged to be substantially perpendicular to the bottom wall; and a second area (425a2) located closer to the upper wall than the bottom wall and arranged to be inclined with respect to the bottom wall, and the second part of the first side wall is arranged to be inclined with respect to the bottom wall.