A concept of apparent energy distribution of sea waves is suggested and named "the outer Dequency spectrum" to distinguish it from the frequency spectrum in common sense In line with the concept, a kind or outer spectrum with three parameters in Cauchy's form is derived theoretically from the joint distribution of wave periods and heights reached previously by the present authors and the Bretschnelder spectrum is rededuced simply in a distinct manner. The foundation of medlanics of the snnilarity between the frequency, spectrum m common sense and the outer one is briefly discussed, in view of the fact that Bretschneider spectrum has a wide application in engineering, the advantages and probabihty of asmg the outer spectrum as a common one are also considered.