A lawn mower comprising: a traveling frame provided with traveling wheels (15, 16); a driving source mounting portion formed in the traveling frame; a driving source mounted in the driving source mounting portion and including a downward rotating shaft; and a lawn mowing unit positioned below the traveling frame and attached to the rotating shaft, a lower end of the driving source mounting portion and a lower end of the driving source being exposed from a bottom surface of the traveling frame, wherein when the traveling frame is viewed from below, a flat annular frame-side seal surface which is positioned so as to surround the lower end of the driving source mounting portion and an annular lawn mowing unit peripheral surface which is positioned so as to surround the frame-side seal surface are formed in the bottom surface of the traveling frame, when the lawn mower is viewed from below, the lawn mowing unit peripheral surface is formed over a range (Ar) from an outer circumferential edge of the frame-side seal surface to at least an outer circumferential edge of the lawn mowing unit, a boundary between the outer circumferential edge of the frame-side seal surface and an inner circumferential edge of the lawn mowing unit peripheral surface is formed into a shape of a step over an entire circumference, characterized in that a waterproof member configured to waterproof a portion between the bottom surface of the traveling frame and the lower end of the driving source is in tight contact with the frame-side seal surface, and at least the inner circumferential edge of the lawn mowing unit peripheral surface is positioned higher than the outer circumferential edge of the frame-side seal surface. a traveling frame provided with traveling wheels (15, 16); a driving source mounting portion formed in the traveling frame; a driving source mounted in the driving source mounting portion and including a downward rotating shaft; and a lawn mowing unit positioned below the traveling frame and attached to the rotating shaft, a lower end of the driving source mounting portion and a lower end of the driving source being exposed from a bottom surface of the traveling frame, wherein when the traveling frame is viewed from below, a flat annular frame-side seal surface which is positioned so as to surround the lower end of the driving source mounting portion and an annular lawn mowing unit peripheral surface which is positioned so as to surround the frame-side seal surface are formed in the bottom surface of the traveling frame, when the lawn mower is viewed from below, the lawn mowing unit peripheral surface is formed over a range (Ar) from an outer circumferential edge of the frame-side seal surface to at least an outer circumferential edge of the lawn mowing unit, a boundary between the outer circumferential edge of the frame-side seal surface and an inner circumferential edge of the lawn mowing unit peripheral surface is formed into a shape of a step over an entire circumference, characterized in that a waterproof member configured to waterproof a portion between the bottom surface of the traveling frame and the lower end of the driving source is in tight contact with the frame-side seal surface, and at least the inner circumferential edge of the lawn mowing unit peripheral surface is positioned higher than the outer circumferential edge of the frame-side seal surface.