A mobile station device includes a receiving section configured to receive a control signal used for changing a value of the transmission power of the mobile station device from a base station, and a control section configured to change, if the transmission power value of the mobile station device is more than a certain threshold value when the control signal has been received, the transmission power value by setting an extent to which the transmission power value is to be changed as a certain extent of change, and, if the transmission power value of the radio mobile station device is less than the certain threshold value when the receiving section received, change the transmission power value by making the extent of change of the transmission power value smaller than the certain extent of change.