An attachment assist device includes an engaging member, an engaged member, a drawing-force generator, biasing members, and rotational members. The engaging member is provided at one of a main unit and a sub unit detachably attached to the main unit. The engaged member is engaged by the engaging member and provided at the other of the main unit and the sub unit. The drawing-force generator generates a drawing force to draw the sub unit into an interior of the main unit. The biasing members are provided at the drawing-force generator to generate biasing forces in different directions. The rotational members are provided rotatably around support points at the drawing-force generator and engaged with the biasing members. The biasing members are serially connected with the rotational members to convert the biasing forces of the biasing members to the drawing force.