  In this paper, we discuss conditions for applying Frobenius's method to
$A$-hypergeometric systems introduced by Okuyama and Saito [3]. We prove that,
when the negative support of a fake exponent $v$ with respect to a generic
weight $w$ is included in the core, we can construct all A-hypergeometric
series with exponent $v$ in the direction $w$ by Frobenius's method. As an
example, we describe the Aomoto-Gel'fand system of type $3 \times 3$ in
details.
