set temperature t 4 in st 27 the program proceeds to st 41 shown in fig1 whereas if the temperature s 2 of the right side wall 1b is lower than the fourth set temperature t 4 in st 27 the first circulating fan 12a is turned off stopped st 28 and the program then proceeds to st 29 in st 29 the temperature s 1 of the left side wall 1a is compared with a value obtained by subtracting the set temperature range tc from the fourth set temperature t 4 if the temperature s 1 of the left side wall 1a is higher than or equal to the value obtained by subtracting the set temperature range tc from the fourth set temperature t 4 in st 29 the program returns to st 25 whereas if the temperature s 1 of the left side wall 1a is lower than the value obtained by subtracting the set temperature range tc from the fourth set temperature t 4 in st 29 the first circulating fan 12a is turned on operated st 30 and the program returns to st 25 referring to fig9 whether or not the second circulating fan 12b is in operation is determined st 31 if the second circulating fan 12b is not in operation in st 31 the program proceeds to st 37.