The present invention is to provide a VP dewatering method being applicable over a wide range, having good handling properties, and being economically efficient and highly energy-saving, which comprises compression-pressuring a dewatered vapor product to elevate a condensation temperature thereof and then re-using the same as a heat source for vaporizing a supply liquid. In dewatering a water-containing organic substance which is a liquid starting material according to a vapor permeation membrane separation method, a dewatered organic substance vapor discharged from a membrane module is pressurized with a compressor to elevate the condensation temperature of the vapor and then the vapor is fed to a vaporizer of said method to collect and use the latent heat of condensation thereof.