  Motivated by the recent discovery of broken four-fold symmetry in the hidden
order phase of URu$_2$Si$_2$[R. Okazaki et al., Science {\bf 331}, 439 (2011)],
we examine a scenario of a spin nematic state as a possible candidate of the
hidden order phase. We demonstrate that the scenario naturally explains most of
experimental observations, and furthermore, reproduces successfully the
temperature dependence of the spin anisotropy detected by the above-mentioned
experiment in a semi-quantitative way. This result provides strong evidence for
the realization of the spin nematic order.
