A corrosion sensor includes an inner electrode formed of a conductive material, the inner electrode including a columnar portion around an axial line as a center thereof and of which an outer circumferential surface is tapered, wherein a diameter of the outer circumferential surface decreases toward one side in an axial direction, an outer electrode formed of a conductive material, the outer electrode including a cylindrical portion around the axial line and of which an inner circumferential surface is tapered, wherein a diameter of the inner circumferential surface decreases toward the one side in the axial direction, and the cylindrical portion is positioned such that the inner circumferential surface radially faces the outer circumferential surface, and an insulation layer formed of an insulation material and in a position across which the outer circumferential surface faces the inner circumferential surface.