PURPOSE: To enhance the safety of a cover open close work and a freedom of usage by installing a locking member to either of a cover and a main body and capable of maintaining a blocked state of the cover and opening the cover by way of this locking member.

CONSTITUTION: A cover device of a pit comprises an engagement device 23 which maintains a blocked state when a cover 15 is blocked and an opening device 35 which opens the cover 15. The engagement device 23 allows a slanting part 27, which turns the cover 15 from an open state to a blocked state, to come in contact with a locking member 30, thereby moving back the engagement member to block the cover 15 and maintaining an engaged state with an engagement member 26. The open device 36, when an operation unit 39 is pushed up with the hand inserted from a notched part 40 of the cover 15, places a pair of operation pieces 42 fixed with a shaft 32 into contact with a contact unit 32 of the engagement member 26, thereby moving back the engagement member 26 and disconnecting the engagement. This construction makes it possible to open or lift up the cover 15 an automatic mode with the degree of elasticity of an elastic member 19 in this state as well.