  The gap structure of Sr$_2$RuO$_4$, which is a longstanding candidate for a
chiral p-wave superconductor, has been investigated from the perspective of the
dependence of its specific heat on magnetic field angles at temperatures as low
as 0.06 K ($\sim 0.04T_{\rm c}$). Except near $H_{\rm c2}$, its fourfold
specific-heat oscillation under an in-plane rotating magnetic field is unlikely
to change its sign down to the lowest temperature of 0.06 K. This feature is
qualitatively different from nodal quasiparticle excitations of a
quasi-two-dimensional superconductor possessing vertical lines of gap minima.
The overall specific-heat behavior of Sr$_2$RuO$_4$ can be explained by
Doppler-shifted quasiparticles around horizontal line nodes on the Fermi
surface, whose in-plane Fermi velocity is highly anisotropic, along with the
occurrence of the Pauli-paramagnetic effect. These findings, in particular, the
presence of horizontal line nodes in the gap, call for a reconsideration of the
order parameter of Sr$_2$RuO$_4$.
