To provide a horizontal member clamp of a guardrail and the guardrail using this clamp, having a structure for restraining projection of a fixing bolt for fixing the horizontal member clamp to a column, without being deformed and broken even if a vehicle collides with a horizontal member or the horizontal member clamp.

This horizontal member clamp is composed of a holding member composed of a holding part 11 for holding the horizontal member and a bracket 12 insertable into a side surface opening part of the column, an installing auxiliary member 13 joinable to a wall surface of a side surface of the column on the opposite side of the opening part, and the fixing bolt 4 to be fixed to the column via the installing auxiliary member in a state of inserting the bracket into the opening part. The horizontal member clamp of the guardrail is characterized in that the bracket is formed in a cylindrical shape or a truncated cone shape in an outside surface, and both the bracket and the installing auxiliary member have a vertical wall having a through-hole or a screw hole for inserting the fixing bolt in the horizontal direction.