To provide a subsidence prevention device for a strut for supporting a guard rail capable of withstanding the weight of a fallen snow even when a large amount of snow falls on the guard rail.

This subsidence prevention device for a guard rail 5 comprises a base 10 having an upper surface with sufficient area. An insertion hole 13 with an inner diameter larger than the outer diameter of the lower part 3c of the strut 3 is formed in the base 10. The device has a bar-like pivotally supporting member 21 of such a size that can be extended through a through hole 4 formed in the intermediate part 3b of the strut 3. The device further comprises a complementing means 25 for vertically integrating the strut 3 with the base 10 by complementing the gap G produced between the upper surface 12 of the base 10 and the bar-like pivotally supporting member 21 fitted to the strut 3 driven into a ground.