To provide a guardrail foundation block which can be set easily in a stable state, can support a guardrail easily and firmly, and facilitates vegetation on the foundation block.

The foundation block 1 includes a lower wall 3 which allows the foundation block 1 to be set easily in a stable state, and a rising wall 5 having a post support hole 4 which easily supports a post 2a of the guardrail 2. The rising wall 5 is so formed as to stand high to provide sufficient vegetation spaces 6 that allow easy and sure arrangement of vegetation 8. The vegetation spaces 6 are formed on both sides of the post support hole 4, where the top of the vegetation spaces 6 are open toward the guardrail 2 and two vegetation spaces 6 are symmetrical in shape and position with regard to the support hole 4. This allows the vegetation 8 to be arranged easily and surely in a fine appearance in a gap between the rising wall 5 and the guardrail 2.