  We propose the application of coronagraphic techniques to the spectroscopic
direct detection of exoplanets via the Doppler shift of planetary molecular
lines. Even for an unresolved close-in planetary system, we show that the
combination of a visible nuller and an extreme adaptive optics system can
reduce the photon noise of a main star and increase the total signal-to-noise
ratio (S/N) of the molecular absorption of the exoplanetary atmosphere: it
works as a spectroscopic coronagraph. Assuming a 30 m telescope, we demonstrate
the benefit of these high-contrast instruments for nearby close-in planets that
mimic 55 Cnc b ($0.6 \lambda/D$ of the angular separation in the K band). We
find that the tip-tilt error is the most crucial factor; however, low-order
speckles also contribute to the noise. Assuming relatively conservative
estimates for future wavefront control techniques, the spectroscopic
coronagraph can increase the contrast to $ \sim 50-130 $ times and enable us to
obtain $\sim 3-6 $ times larger S/N for warm Jupiters and Neptunes at 10 pc
those without it. If the tip-tilt error can be reduced to $\lesssim 0.3$ mas
(rms), it gains $\sim 10-30$ times larger S/N and enables us to detect warm
super-Earths with an extremely large telescope. This paper demonstrates the
concept of spectroscopic coronagraphy for future spectroscopic direct
detection. Further studies of the selection of coronagraphs and tip-tilt
sensors will extend the range of application of the spectroscopic direct
detection beyond the photon collecting area limit.
