With the aid of Abrikosov's quasifermion and the method of Green functions, a diagram analysis is made of the theory of Yosida, Okiji and Yoshimori on the bound state of conduction electrons coupled with a localized spin. The bound state is represented by an isolated pole of the Green function which describes the pair of one electron and one quasifermion injected into the Fermi sea of the free electron gas. When the most divergent vertex corrections and also the regular self-energy corrections are included, the secular equation to determine the isolated pole takes precisely the same form as Yoshimori has found. It is thus confirmed that the theory is exact in the weak coupling limit and at zero temperature.