A lawn mower comprising: a blade disk rotatable about a rotation shaft extending in an up-down direction; and a plurality of cutter blades mounted on the blade disk, wherein the blade disk integrally includes a disk base section of a flat disk shape mounted on the rotation shaft, a plurality of blade mounting sections having corresponding ones of the cutter blades mounted thereon, and a circumferential high-rigidity section for increasing rigidity of the blade disk between the blade mounting sections, the blade mounting sections are located near the outer circumferential edge of the blade disk and located on, and spaced from each other along, an imaginary reference circle (Cs) concentric with a rotation center of the blade disk, and the circumferential high-rigidity section is located along the reference circle (Cs) in such a manner as to interconnect the plurality of blade mounting sections along the reference circle, the circumferential high-rigidity section being formed in a generally concave-convex configuration by being depressed and protruded in a facewise direction of the blade disk, wherein the blade disk further integrally includes a plurality of radial rigid sections for increasing rigidity of the blade disk between the disk base section and the blade mounting sections, the radial rigid sections are located between the disk base section and the blade mounting sections, and the radial rigid sections are formed in a generally concave-convex configuration by being depressed or protruded in a disk surface direction, and the circumferential high-rigidity section comprises a plurality of depressed portions and each of the blade mounting sections is surrounded by a corresponding depressed portion, characterized in that the circumferential high-rigidity section additionally comprises a plurality of beads formed continuously and alternately with the depressed portions along the entire circumference of the reference circle (Cs). a blade disk rotatable about a rotation shaft extending in an up-down direction; and a plurality of cutter blades mounted on the blade disk, wherein the blade disk integrally includes a disk base section of a flat disk shape mounted on the rotation shaft, a plurality of blade mounting sections having corresponding ones of the cutter blades mounted thereon, and a circumferential high-rigidity section for increasing rigidity of the blade disk between the blade mounting sections, the blade mounting sections are located near the outer circumferential edge of the blade disk and located on, and spaced from each other along, an imaginary reference circle (Cs) concentric with a rotation center of the blade disk, and the circumferential high-rigidity section is located along the reference circle (Cs) in such a manner as to interconnect the plurality of blade mounting sections along the reference circle, the circumferential high-rigidity section being formed in a generally concave-convex configuration by being depressed and protruded in a facewise direction of the blade disk, wherein the blade disk further integrally includes a plurality of radial rigid sections for increasing rigidity of the blade disk between the disk base section and the blade mounting sections, the radial rigid sections are located between the disk base section and the blade mounting sections, and the radial rigid sections are formed in a generally concave-convex configuration by being depressed or protruded in a disk surface direction, and the circumferential high-rigidity section comprises a plurality of depressed portions and each of the blade mounting sections is surrounded by a corresponding depressed portion, characterized in that the circumferential high-rigidity section additionally comprises a plurality of beads formed continuously and alternately with the depressed portions along the entire circumference of the reference circle (Cs).