PURPOSE: To continuously measure the density of a volatile component existing in a liquid exactly and stably for a long period of time, by collecting a volatile component in a liquid, controlling the feed pressure of carrier gas passing through one side of a liquid-repelling porous partition body, and detecting the density of the volatile component.

CONSTITUTION: Pressure of the gaseous phase side is controlled so that a pressure difference ΔPkg/cm2 between a liquid to be measured of one side placing a liquid repelling porous partition body between them, and its opposite gaseous phase part becomes ≤ΔP=0.02σcosθ/R(kg/cm2), a value which is decided by surface tension σ dyne of a liquid to be measured, a contact angle θ which is generated when the liquid to be measured contacts with a porous partition material, and the hole diameter R(μ) corresponding to a microscopic hole of the porous partition body. Moreover, the density of a volatile component contained in this liquid to be measured is measured by leading the gas from gaseous phase to a detector intermittently or continously, and measuring and detecting the density of a volatile component which is transmitted to gaseous phase from the liquid to be measured.