To provide a closed section blocking structure capable of reducing the number of parts and the mounting man hour.

In the closed section blocking structure 10, a reinforcement 26 in a front pillar 20 forms two closed sections 28 between an inner panel 22 and an outer panel 24. A blocking member 32 mounted on the reinforcement 26 blocks each closed section 28 by foaming each forming material 38 provided on each closed section 28 side. When one mounting work of one blocking member 32 is performed from the closed section 28 side on the cabin 16 side, each closed section 28 is blocked, and the number of parts and the mounting man hour can be reduced. In addition, by adjusting the amount and the expansion ratio of the foaming material 38 on each closed section 28 side corresponding to the size of each closed section 28, the amount and the expansion ratio of each foaming material 38 can be optimally set to the size of each closed section.