A mobile communication system which is capable of, when carrying out mobile communication using a shared channel, increasing in efficiency of transmission timing of the data transmission rate request value to prevent wasteful power consumption and hence reduce power consumption. A mobile station apparatus of the mobile communication system measures CIR of the received signal from a base station apparatus at a CIR measuring section, and decides the data transmission rate request value corresponding to the measured CIR value at a rate request value deciding section. Also, it detects an error of the received signal at a CRC section, and, when no error is found, calculates a difference between the average data transmission rate from a base station apparatus and the data transmission request value at a rate request value transmission controlling section. Then, it transmits the data transmission rate request value to the base station apparatus only when the obtained difference is larger than a threshold value.