A spectrum spread communication system having a communication base unit which communicates with communication extension units (5a to 5f), wherein each of the communication extension units has: a spread code generator adapted to switch and selectively output a spread code (Scai, Scbi) in each frame; and a spread modulation unit adapted to perform spread modulation of frame-structured transmission data by using the phase of the spread code from the spread code generator, the spread code generator being adapted to switch the spread code at the next frame and the extension unit being adapted to transmit the spread-modulated signal to a transmission line, the communication base unit has: a reference code generating unit adapted to generate a plurality of reference codes (Scai and Scbi) corresponding to the plurality of spread codes, spread demodulation units (43a and 43b) in respective spread demodulation systems (a, b), the spread demodulation units being adapted to perform spread demodulation of the spread-modulated signal received via the transmission line by using the reference codes, a code phase synchronization detection unit adapted to control the phase of the reference codes from the reference code generating unit so that the spread-modulated signal is synchronized in phase with any one of the reference codes; a frame synchronization detection unit -46) adapted to output a frame synchronization signal on the basis of timing of switching from a state in which the spread-modulated signal is synchronous with one reference code, to a state in which the spread-modulated signal is synchronous with another reference code, and a selection unit adapted to selectively output, on the basis of the frame synchronization signal, signals demodulated by the spread demodulation units. a spread code generator adapted to switch and selectively output a spread code (Scai, Scbi) in each frame; and a spread modulation unit adapted to perform spread modulation of frame-structured transmission data by using the phase of the spread code from the spread code generator, the spread code generator being adapted to switch the spread code at the next frame and the extension unit being adapted to transmit the spread-modulated signal to a transmission line, the communication base unit has: a reference code generating unit adapted to generate a plurality of reference codes (Scai and Scbi) corresponding to the plurality of spread codes, spread demodulation units (43a and 43b) in respective spread demodulation systems (a, b), the spread demodulation units being adapted to perform spread demodulation of the spread-modulated signal received via the transmission line by using the reference codes, a code phase synchronization detection unit adapted to control the phase of the reference codes from the reference code generating unit so that the spread-modulated signal is synchronized in phase with any one of the reference codes; a frame synchronization detection unit -46) adapted to output a frame synchronization signal on the basis of timing of switching from a state in which the spread-modulated signal is synchronous with one reference code, to a state in which the spread-modulated signal is synchronous with another reference code, and a selection unit adapted to selectively output, on the basis of the frame synchronization signal, signals demodulated by the spread demodulation units. a reference code generating unit adapted to generate a plurality of reference codes (Scai and Scbi) corresponding to the plurality of spread codes, spread demodulation units (43a and 43b) in respective spread demodulation systems (a, b), the spread demodulation units being adapted to perform spread demodulation of the spread-modulated signal received via the transmission line by using the reference codes, a code phase synchronization detection unit adapted to control the phase of the reference codes from the reference code generating unit so that the spread-modulated signal is synchronized in phase with any one of the reference codes; a frame synchronization detection unit -46) adapted to output a frame synchronization signal on the basis of timing of switching from a state in which the spread-modulated signal is synchronous with one reference code, to a state in which the spread-modulated signal is synchronous with another reference code, and a selection unit adapted to selectively output, on the basis of the frame synchronization signal, signals demodulated by the spread demodulation units.