Main consequences of the two-meson theory by Marshak and Bethe on the one hand and that by Sakata are compared with experiments concerning $\pi$- and $\mu$-mesons. It is shown that the pair type interaction between $\pi$-mesons and nucleons in "M. B." theory contradicts with frequent occurrence of stars produced by $\pi$-mesons, whereas the assumption in Sakata theory that $\pi$-mesons with spin 0 or 1 are responsible for nuclear forces does not. Although the smaller range for the nuclear forces thus obtained from the latter theory is not at variance with the high energy neutron-proton scattering experiment, deuteron quadrupole moment cannot be accounted for by a $\pi$-meson field alone, so that the admixture of another meson field with larger range is necessitated. Both $\pi -\mu$-decay and $\mu$-nuclear capture can be consistently accounted for by assuming spin ½ for $\mu$-mesons as in Sakata theory. However, nuclear $\beta$-decay and $\mu -\beta$-decay have to be considered as direct processes as in Fermi's theory of $\beta$-decay, instead of indirect processes through virtual emission and absorption of $\pi$-mesons as assumed usually in the meson theory.

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