To provide a drop preventing structure capable of reducing the fear that a second member such as a unit fitting or the like suddenly drops from a first member such as a casing.

In this drop preventing structure, the second member fastened to the lower side of the first member by a screw member suddenly drops from the first member in removing the second member from the first member. A screw hole for screw-engaging the screw member is formed to penetrate the first member, and a through-hole with a diameter larger than that of the screw member is formed in the second member. A relief part with a diameter smaller than the inside diameter of the screw hole is formed in the middle part of the screw member to form a forward screw part and a backward screw part, and the backward screw part is screw-engaged with the screw hole to fasten the first member and the second member.