  In this paper, we study Giambelli type formula in the KP and the BKP
hierarchies. Any formal power series $\tau(x)$ can be expanded by the Schur
functions. It is known that $\tau(x)$ with $\tau(0)=1$ is a solution of the KP
hierarchy if and only if the coefficients of this expansion satisfy Giambelli
type formula. It is proved by using Sato's theory of the KP hierarchy. Here we
give an alternative proof based on the previously established results on the
equivalence of the addition formulae and the KP hierarchy without using Sato's
theory. This method of the proof can also be applied to the case of the BKP
hierarchy.
