Abstract The apparent temperature of the quiet sun measured at radio frequencies is calculated with varying value of the electron density in the corona and chromosphere to see how it changes according to a sunspot cycle. The electron temperature in the chromosphere is assumed to increase exponentially with height until it reaches a constant value in the corona. The electron density in the corona is supposed to vary by a factor of two during a whole sunspot cycle without any change in the form, but in the chromosphere is assumed to vary in two different manners. The observations by Covington at 10.7 cm during 5 years seem to fit the calculated curve, while the decrement of the apparent temperature as given by Christiansen and Hindman on five frequencies between two periods seems not so well fitted, althogh the existence of a maximum of the variation of the apparent temperature at a frequency of about 1000 Mc/s is confirmed both from theory and observation.