The present invention relates to an apparatus for monitoring a tire pressure. Provided is a tire pressure monitoring apparatus including: a radius analyzing unit which calculates a radius analysis value using a relative speed difference and an average speed calculated from wheel speeds of the wheels mounted on the vehicle; a regression equation calculating unit which calculates a first regression equation for the calculated radius analysis value and first driving information and a second regression equation for the calculated radius analysis value and second driving information; a mass calculating unit which calculates an additional mass of the vehicle; a calibration unit which corrects the calculated radius analysis value using a combination of the calculated first and second regression equations and the calculated additional mass; and a low pressure determining unit which determines a low pressure of a tire mounted on the vehicle using the corrected radius analysis value.