We developed a cylindrical resonator-type microwave (MW) heating reactor with the function of real-time monitoring of dielectric properties. Using automatic detection and tracking functions of the resonance frequency shift with a variable-frequency semiconductor generator, we estimated qualitative variations of the relative dielectric constant and dielectric loss factor for continuous heating of ethylene glycol without connecting a network analyzer. Additionally, we undertook resin-pellet drying using this reactor with a weight measurement function. The drying state of the resin pellets was estimated using resonance frequency variation. Moreover, differences of moisture reduction and resin-pellet melting were distinguished in situ from variation of the resonance curve corresponding to the S21 parameter.