PURPOSE: To easily prevent cooling water in a pressurizer from overflowing by placing between the side and bottom boards of the pressurizer partition boards whose upper ends stick out upward from the surface of cooling water and installing between the partition boards porous boards where numerous cooling water passing holes are drilled.

CONSTITUTION: A water surface pitching prevention board 4 is constituted by installing partition boards 6 whose upper ends stick out upward from the surface of cooling water at equal intervals and radically between side boards 2 and bottom boards 3 of a pressurizer placed in the axial center inside a pressure vessel of a nuclear reactor and laying horizontally like a bridge between the partition boards 6 horizontally porous boards 7 where numerous passing holes 8 are drilled. Even when a vacuum vessel tilts, the porous boards 7 of the prevention board 4 controls the flow of cooling water toward the tilting direction and prevents the cooling water from overflowing the partition boards 6 into divided parts in the tilting direction. During the normal operation, moreover, the original functions of the pressurizer are not damaged because the cooling water heated by a heater easily moves through passing holes toward the side of the water surface (upward) and jets of cooling water are poured directly over it.