The total solar fluxes at 1, 2, 3.75 and 9.4 GHz have been observed continuously from 1957 to 1994 at Toyokawa, from 1994 at Nobeyama, Japan. We examined the multi-frequencies and long-term dataset, and found that monthly standard deviation of total solar flux in microwave can indicate an actual minimum state of solar activity rather than sunspot number. Therefore, we define "radio solar minimums" from the value. Comparing the microwave spectrums of the radio solar minimums of Cycle 20∼24, we found that they match within 7%. It shows that the average atmospheric structure above transition region in quiet sun didn’t vary during the half a century, and suggest that the energy input for atmospheric heating from sub-photosphere to corona didn’t change in quiet sun though the strength of solar cycles variation changed significantly.