  The specific heat oscillation in the mixed state of type II superconductors
is studied theoretically when rotating field within a plane containing gap
minimum and maximum. The calculations are performed microscopically by solving
quasi-classical Eilenberger equation for vortex lattices. The field dependence
of the oscillation amplitude can discriminate between the nodal and anisotropic
gap with a finite minimum and the oscillation phase gives the gap minimum
position on the Fermi surface. These also provide a way to separate out the
anisotropic behavior due to the Fermi velocity.
