To provide, by a simple structure, a constant force expansion device expanding/retracting when forces exceed specified forces.

In this constant force expansion device A expanding/retracting when the pushing force and a pulling force exceed the specified forces, a compression spring 5 to which a specified pre-compressive force is given is pressingly held between a pulling plate 3 and a pushing plate 4 so that a pushing bar 6 inserted into one end opening of a cylinder 1 through a locking member 2 can be engaged with the pushing plate 4 in the pushing direction and a connection bar 7 connected to the pulling plate 3 can be passed through the pushing plate 4 and engaged with the pushing bar 6 in the pulling direction.