To secure sufficient sealing performance between a detecting part holder and a casing even to large temperature changes.

The rotation detecting sensor P detects a change in magnetic field due to the rotation of a body to be detected B by a detecting part (a), converts it into an electric signal, and outputs it. The detecting part (a) is mounted to the holder 2, and the detecting part (a) is inserted in the tubular casing 1 with a lid from its opening. The holder 2 is fitted to an opening part of the casing 1. The entire circumference between the opening edge of the casing 1 and the holder 2 is blocked by a liquid gasket 12, and the entire circumference of the opening part of the casing 1 and an exposed part of the holder 2 is coated with a resin 4 across both of them. The liquid gasket 12 smoothly enters a gap with satisfactory sealing performance as it is in liquid form, and workability is satisfactory as its liquid component is fast setting and speedily volatilizes. In addition, it smoothly follows temperature change to secure sealing performance and does not melt or leak out as it forms an elastic seal part after volatilization.