An apparatus for estimating a tire pressure condition amount of a tired wheel mounted in a vehicle is provided with a wheel speed sensor (10) for detecting a vehicle wheel speed representing an angular speed of the tired wheel and for outputting a vehicle wheel speed signal in response thereto and an estimating unit (20) for estimating the tire pressure condition amount based upon the vehicle wheel speed signal outputted from the wheel speed sensor. The estimating unit includes a vehicle condition amount detecting device (40) for detecting a vehicle condition amount and for outputting a vehicle condition amount signal in response thereto. An estimation of the tire pressure condition amount by using the vehicle wheel speed signal is normalized based upon the vehicle condition amount signal outputted from the vehicle condition amount detecting device.