To provide a guardrail foundation structure capable of preventing an impact load generated when an automobile is collided with a guardrail from being transmitted to a retaining wall while a specified width of a road is secured by effectively utilizing an upper space above the upper end face of the retaining wall.

Foundation blocks 5 for guardrail are laid down in a connected state along a shoulder 7 having an edge part protected by the retaining wall 6 so as to cover the upper end face 9 of the retaining wall 6. A clearance 8 is formed between the upper end face 9 of the retaining wall 6 and the lower surface 10 of the foundation blocks 5. The vertical width of the clearance 8 is set to 10 to 30 mm. Even if the front portion of the foundation blocks 5 is deformed downward by the impact load generated when the automobile is collided with the guardrail, the deformed part is not brought into contact with the upper end face 9 of the retaining wall 6.