  We have investigated the field-angle variation of the specific heat C(H, phi,
theta) of the heavy-fermion superconductor UPt3 at low temperatures T down to
50 mK, where phi and theta denote the azimuthal and polar angles of the
magnetic field H, respectively. For T = 88 mK, C(H, theta=90) increases
proportionally to H^{1/2} up to nearly the upper critical field Hc2, indicating
the presence of line nodes. By contrast, C(H, theta=0) deviates upward from the
H^{1/2} dependence for (H/Hc2)^{1/2} > 0.5. This behavior can be related to the
suppression of Hc2 along the c direction, whose origin has not been resolved
yet. Our data show that the unusual Hc2 limit becomes marked only when theta is
smaller than 30. In order to explore the possible vertical line nodes in the
gap structure, we measured the phi dependence of C in wide T and H ranges.
However, we did not observe any in-plane angular oscillation of C within the
accuracy of dC/C~0.5%. This result implies that field-induced excitations of
the heavy quasiparticles occur isotropically with respect to phi, which is
apparently contrary to the recent finding of a twofold thermal-conductivity
oscillation.
