To provide a precast concrete guard fence installable in a construction limit of a line width, capable of further reducing cost without requiring to assemble a scaffold under a viaduct, and capable of shortening an installing construction period.

This precast concrete guard fence 1 is arranged over ground coverings 11a and 11a of roadbeds 11 and 11 of the right-left viaducts for upper-lower travel lines, and has bilaterally symmetrical side surfaces 4 and 4 composed of a lower surface 2 of a maximum width, an upper surface 3 of a minimum width, a moderately inclined lower inclined face 4a and a steeply inclined upper inclined face 4b. Its lower surface 2 has a straddling-shaped recessed groove 5 in a central part in its longitudinal direction. Both-side lower surfaces 2 of the recessed groove 5 are formed as a horizontal plane 6, and a vibration absorbing rubber plate 9a is arranged and installed in a lower part of these horizontal planes 6. These precast concrete guard fences 1 are composed of a continuously connecting body 1b of one row formed by integrally connecting a plurality of precast concrete blocks 1a of 3 to 5 pieces.