To provide a method for mounting an underground shutoff plate to a penetrating frame keeping an existing shutoff plate and a penetrating shutoff plate in engagement with each other even if an external force is applied near the engaging portions of the plates when the shutoff plates are caused to penetrate the ground.

The method for mounting the underground shutoff plate to the penetrating frame is characterized in that when the underground shutoff plate having a male engaging part 2 and a female engaging part 3 on both sides of a sheet body part 1 along its vertical direction is detachably mounted to one side of the penetrating frame 6, a tolerance X for projection from the end of the penetrating frame 6 facing the female engaging part 8 of the underground shutoff plate ranges from half to twice the thickness Y of the penetrating frame.