Vapor-liquid equilibria of water-ethanol-1-propanol system containing a minute amount of furfural were measured at atmospheric pressure by use of an Othmer-type still. The experimental results are represented by two triangular diagrams against the liquid compositions of the major components (water, ethanol and 1-propanol), in which one diagram shows the vapor compositions of ethanol and 1-propanol, and another diagram shows the equilibrium ratio of the trace component (furfural). The curved surface of the equilibrium ratio of the trace component exhibits a bow-shaped downward plane in the water-rich area and shows a ridge in the neighborhood of the water-1-propanol side. In the alcohol (ethanol and 1-propanol)-rich area it exhibits a saddle-shaped face.