Reference phase is a radio communication system using orthogonal M-ary modulation is determined, so that a coherent detection system for a CDMA radio communication system is readily achieved. The determination of reference phase is performed by the step of determining a reference phase in a radio communication system, which uses orthogonal M-ary codes comprising the steps of determining the correlation values of an in-phase component and quadrature component corresponding to respective Hadamard matrices resulting from fast Hadamard transform or inverse Hadamard transform of a signal, which possesses said in-phase component and quadrature component, and which is sent from the sending end by phase shift keying an orthogonal M-ary code computing the sum of the squares of the respective correlation values of said in-phase component and quadrature component and selecting either said fast Hadamard transform or inverse Hadamard transform output, which outputs a maximum value of said computed sum of the squares.