The examples which must not be treated by the perturbation are given, and it is shown that there exist some second kind interactions which become to the first kinds, if the uses of the perturbation are avoided. Especially, the pseudoscalar theory with the vector coupling after the Dyson transformation belongs to this, although it was proven only for the vanishing meson mass. At last, the Green functions in the scalar meson theory with the vector coupling are obtained exactly.