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. The mobile station apparatus of the mobile communication system measures CIR of the received signal from the base station apparatus at the CIR measuring section, and decides the data transmission rate request value corresponding to the measured CIR value at the rate request value deciding section. Also, it detects an error of the received signal at the CRC section, and, when no error is found, calculates a difference between the average data transmission rate from the base station apparatus and the data transmission request value at the 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.