In this paper we propose a direct sequence spread spectrum (DS/SS) modulation method which employs Gaussian-filtered minimum shift keying (GMSK) and permits simple code acquisition. A transmitter which includes a conventional GMSK modulator and pseudo-noise (PN) code generator can achieve the proposed modulation method. The received signal can be demodulated by four-phase correlator which can obtain the correlation value of received signal even if phase difference exists between the transmitter and the receiver. The modulation method employs phase-shift-keying (PSK) by modulating the phase of transmitted PN code for data transmission. We carried out hardware experiments and the measured bit error performance ensures the validity of this modulation method. Then we designed and developed a demodulator LSI which is applicable to a modulation method such as the DS/GMSK/PSK. The LSI is suitable for demodulation of spreadspectrum signal which can be demodulated by four-phase correlator.