  Future high-contrast imaging spectroscopy with a large segmented telescope
will be able to detect atmospheric molecules of Earth-like planets around G- or
K-type main-sequence stars. Increasing the number of target planets will
require a coronagraph with a small inner working angle (IWA), and wide spectral
bandwidth is required if we enhance a variety of detectable atmospheric
molecules. To satisfy these requirements, in this paper, we present a
coronagraphic system that provides an IWA less than 1$\lambda_0 / D$ over a
moderate wavelength band, where $\lambda_0$ is the design-center wavelength and
$D$ denotes the full width of the rectangular aperture included in the
telescope aperture. A performance simulation shows that the proposed system
approximately achieves a contrast below $10^{-10}$ at 1$\lambda_0 / D$ over the
wavelengths of 650--750nm. In addition, this system has a core throughput
$\geq$ 10\% at input separation angles of $\sim$ 0.7--1.4$\lambda_0 / D$; to
reduce telescope time, we need prior information on the target's orbit by other
observational methods to a precision higher than the width of the field of
view. For some types of aberration including tilt aberration, the proposed
system has a sensitivity less than ever-proposed coronagraphs that have IWAs of
approximately $1\lambda_0/D$. In future observations of Earth-like planets, the
proposed coronagraphic system may serve as a supplementary coronagraphic system
dedicated to achieving an extremely small IWA.
