  To investigate a mysterious superconducting state of URu_2Si_2 embedded in
the so-called hidden order state, the lower critical field H_{c1} is precisely
determined down to 55 mK for H || a and H || c. For this purpose, the
positional dependence of the local magnetic induction is measured on ultraclean
single crystals (T_c = 1.4 K) with residual resistivity ratio exceeding 700. We
find that the temperature dependence of H_{c1} significantly differs from that
of any other superconductors. The whole H_{c1}(T) for H || a are well explained
by the two superconducting gap structures with line and point nodes, which have
been suggested by the recent thermal conductivity and specific heat
measurements. On the other hand, for H || c, a change of slope with a distinct
kink in H_{c1}(T), which cannot be accounted for by two gaps, is observed. This
behavior for H || c sharply contrasts with the cusp behavior of H_{c1}(T)
associated with a transition into another superconducting phase found in UPt_3
and U_{1-x}Th_xBe_{13}. The observed anomalous low-field diamagnetic response
is possibly related to a peculiar vortex dynamics associated with chiral
domains due to the multicomponent superconducting order parameter with broken
time reversal symmetry.
