To reduce a force required to open a back door by moving a hinge to a vehicle body such that a distance from the hinging center P of an opening/ closing body which can freely be open or closed around a hinge to the line of action of reactive force which the stay damper 3 gives the opening/closing body is in a specified state.

An opening/closing device of a back door 1 of a car as an opening/closing body has a hinge 2 for rotatably supporting a back door 1 and a stay damper 3 for giving the back door 1 an opening moment and the center P of the hinge 2 is the center of the hinging center of the back door 1 to a vehicle body 10. The hinge 2 is moved to the vehicle body 10 such that the distance from the hinging center P to the line of action of reactive force which the stay damper 3 gives the back door 1 when the back door 1 is opened from the closed position against the closing moment by the self-weight of the back door 1 is larger than the distance when the back door 1 is closed.