Let f be an infinitely divisible
characteristic function whose spectral functions are absolutely continuous
functions with almost everywhere continuous derivatives. Some necessary
conditions that f belong to the class I0 of the infinitely divisible characteristic
functions without indecomposable factors have been obtained by Cramér,
Shimizu and the author and a sufficient condition that f belong to I0 has been
given by Ostrovskiy. In the present work, we prove that the condition of
Ostrovskiy is not only a sufficient, but also a necessary condition that f belong to
I0.