  When magnetic field is applied parallel to the ab plane in d_{x^2-y^2}-wave
superconductors, the transition of stable vortex lattice structure, spatial
structure of local density of states, and specific heat oscillation by rotation
of magnetic field orientation are investigated by quantitative calculations
based on the selfconsistent Eilenberger theory. We estimate how the vortex
state changes depending on the relative angle between the node-direction of the
superconducting gap and magnetic field orientation. To reproduce the
sign-change of specific heat oscillation observed in CeCoIn_5, our study is
done by including strong paramagnetic effect. The quantitative theoretical
calculations give decisive information to analyze the experimental data on the
field-angle dependence, and establish the angle-resolved specific heat
experiment as a spectroscopic means to identify the node-position of the
superconducting gap.
