This study was performed to suggest the basic information for the development of covering method for increasing the effect of heat conservation in the unheated vinyl houses. Double and single covered vinyl houses were constructed and the inside temperature variation of the double covered vinyl house was compared to the single covered vinyl house. The results obtained for the vinyl houses were summarized as follows ; 1. With the outside temperature of $-1.0^{circ}C{sim}7.5^{circ}C$ and the solar radiation of maximum $1.02MJ/m^2$, the double covered vinyl house was shown $1.2^{circ}C$ higher than the single covered vinyl house in the daily mean of inside temperature. 2. The inside temperature of the double covered vinyl house was $2.3^{circ}C$ higher in the daytime and $1.2^{circ}C$ higher in the nighttime than that of the single covered vinyl house. 3. The inside temperature of the vinyl house were expressed by the formular (1)-(6) with the outside temperature and by the formular (7)-(10) with the solar radiation. 4. In the effect of heat conservation based on the outside temperature, the double covered vinyl house was 1.2 times in the daytime and 1.76 times in the nighttime in comparison with the single covered vinyl house.