A communication semiconductor integrated circuit device includes an RFVCO and a TXVCO and is formed over one semiconductor substrate, and has a first operation mode (idle mode) which does not perform transmission and reception, a second operation mode (warmup mode) which performs a preparation prior to the start of transmission or reception, and a third operation mode (transmission or reception mode) which performs transmission or reception. In the first operation mode, two oscillators are deactivated, and the operation of selecting a frequency band to be used in at least the TXVCO which generates a transmit signal, is performed in the second operation mode. In the communication semiconductor integrated circuit device, a high frequency signal generator which generates a signal having a desired frequency with an oscillation signal outputted from a crystal oscillator as a reference signal, a reception system circuit (RXC) including a frequency converter, and a transmission system circuit (TXC) including the TXVCO which generates the transmit signal, are respectively formed in different semiconductor regions isolated by insulators.