To provide a guardrail fixing device for fixing, to a fixing post, a guardrail for separation of a roadway from a sidewalk.

The guardrail fixing device includes, as a set: a hexagonal column shape base 1 connected to a bolt part 4 provided with a male right screw 3 on one side part via a projection part 2 and also connected to a bolt part 6 provided with a male left screw 5 on the other side part; a fixing nut 8 to be screwed with the male right screw 3 provided to the bolt part 4 in addition to the base; and a fixation adjusting nut 10 provided with a projection part 11 to be screwed with the bolt part 6 having the male left screw 5.