A signal processing device such as an RFIC having a function of holding therein control data, enabling stable operation of an integrated circuit because of suspension of power supply during the intermittent reception suspension period in the standby mode, and enabling further improvement of power saving. A portable communication terminal device and a radio communication system are also provided. The signal processing device has an RFIC unit (12) and an ASIC unit (13) connected to the RFIC unit (12) via an input/output buffer circuit (140), making control such that power supply to the RFIC unit (12) is suspended during the reception suspension period during which signaling information is not received in the standby mode and power is supplied to the RFIC unit (12) when signaling information is received, and including an SPI therein. The ASIC unit (13) makes a control simultaneously with suspension of the power supply to the input/output buffer circuit (140), fetches a register value from a control register (34) before the power supply to the RFIC unit (12) is suspended, holds the register value in the SPI, and transfers the held register valued to the RFIC unit (12) and the control register (34) when the power supply is restarted.