A density calibration using microwave reflectometry for a Thomson scattering measurement is proposed. The calibration can be performed in parallel with a plasma experiment. A compact and simple FM-type reflectometer is preferable due to the small space around the Thomson scattering device. A Bayesian estimation method is also proposed for the derivation of the calibration factor. The Bayesian method requires fewer data from the reflectometer than an average method for an accurate density calibration factor, which is confirmed by a Monte Carlo simulation.