PURPOSE: To completely thaw the snow of a roof, by spraying water within a water reservoir with a water-sprayer on the roof due to the pump operation through detection of a snow-sensor and by storing the thawed snow water in the water reservoir via collecting troughs.

CONSTITUTION: A snow-sensor 50 transmits an output signal S by detection of falled snow to open an electromagnetic valve 43 and to start a pump 40. The pump 40 sucks the water within a water reservoir 30 to feed it to spray pipes 10. Next, the water is spraied from spray-nozzles 11 of the pipes 10 all over the roof R and hence, falled snow on the roof R is melted before it accumulates on the roof R and the thawed snow flowing down the roof R is collected by troughs 21 under the eaves of water collecting troughs 20 and is stored in the reservoir 30 through a water pipe T1. The water within the tank 30 is spraied again from the pipe 10 by the pump 40.