To provide a method of fixing a post into a burying hole while preventing corrosion of a steel pipe used as the post of a guardrail.

A base part 11 of the steel post 1 is buried and fixed into the ground after applying anticorrosion treatment to the base part. As the anticorrosion treatment, the base part is covered with a cylindrical plastic sleeve 6 through an adhesive sheet 5, and a cap 7 made of rubber with high weather resistance is put on the upper part of the cylindrical sleeve. The inner diameter of the rubber cap is smaller than the outer diameter of the post, and when put on, the upper end of the rubber cap is directly brought into close contact with the post to prevent infiltration of rainwater between the post and the cylindrical sleeve while holding down the upper part of the cylindrical sleeve. Burying is carried out by putting the corrosion-treated post in the burying hole provided beforehand, filling sand 4 around leaving 1-5 cm from the ground surface, and then filling cement mortar 3 on top of the sand for fixing.