On the basis of the three premises of Stoner's collective electron theory of ferromagnetism, appropriate energy band diagrams are constructed. These diagrams vividly show the physical situation involved in Stoner's theory. With the help of diagrams, Stoner's criterion for occurrence of ferromagnetism, kθ′/ε0=ξ>2/3, and for complete magnetization, ξ≥2-1/3, can be deduced in a rather simple and straightforward way. With the help of diagrams, the transfer of electrons from d-band to s-band due to splitting of d-band in the ferromagnetic state is considered, and is estimated for nickel at 0°K to be about 1%.