An attachment assist device includes an engagement member provided in one of a main body and a retractable unit, a catch portion provided in the other of the two, to engage the engagement member when the retractable unit is positioned at a drawing start position, and a drawing unit provided in the main body, to draw the retractable unit to a drawing completion position in the main body against multiple resistive forces acting on the retractable unit. On a projection plane on which the retractable unit is projected in an attachment direction, an engagement position where the engagement member engages the catch portion is disposed inside a rectangular area defined by two opposing corners respectively positioned at points of action of two of the multiple resistive forces acting on the retractable unit while the retractable unit moves from the drawing start position to the drawing completion position.