To shorten the drying time and to prevent the drying energy loss.

A mixture ratio sensor 8 is provided at the dry air inlet side of the low temperature side gas of a rotary regenerative type heat exchanger 3, a mixture ratio sensor 9 is provided at the dry air outlet side of a high temperature side gas, and the mixture ratios at the inlet and outlet sides are measured. The ratio to be measured by the sensor is kg (water)/kg (air) according to JIS Z 8806, and obtained by the calculation by measuring the absolute humidity and the temperature. The measured mixture ratio is obtained with the signal by a converter, and input to a calculator. The calculator oscillates a dry end signal when (outlet side mixture ratio)-(inlet side mixture ratio) ≤1g/kg (air) is satisfied, and oscillates a control signal to a forced draft fan 6 or a vapor type air preheater 7 so that the relative humidity of the outlet side becomes about 90% RH. It has utilities such as a boiler 1, a denitrifier 2, a rotor 3a, an electric dust collector 4, and a stack 5.