Moisture Detection Device

A plurality of minute projections ( 10 - 2 ) are provided on a mirror surface ( 10 - 1 ) of a mirror ( 10 ). The shape of a projection ( 10 - 2 ) is not limited to a conical shape, and may be a cylindrical shape, semispherical shape, quadratic prism shape, and the like. Furthermore, the shape may be a polyhedral shape with many surfaces. With a reduction in the temperature of the mirror ( 10 ), water vapor contained in a gas to be measured condenses on the mirror surface ( 10 - 1 ). In this case, because of the minute projections ( 10 - 2 ) on the mirror surface ( 10 - 1 ) of the mirror ( 10 ), the condensation is promoted by the projections ( 10 - 2 ) serving as nuclei. This facilitates condensation even at low dew points and improves responsiveness. Further, the sizes of condensed dew drops do not easily vary relative to variation in the flow rate of the gas to be measured, and this makes the equilibrium state of condensation unlikely to break, increasing measurement accuracy.