  For more than two decades, the nonmagnetic anomaly observed around 17.5 K in
URu2Si2, has been investigated intensively. However, any kind of fingerprint
for the lattice anomaly has not been observed. Therefore, the order has been
called "the hidden order". One simple answer to why the hidden order is still
hidden is presented from the space group analysis. The second order phase
transition from I4/mmm (No. 139) to P4_2/mnm (No. 136) does not need any kind
of lattice distortion in this system, and allows the NQR frequency at Ru-site
unchanged. It is compatible with O_{xy}-type anti-ferro quadrupole ordering
with Q=(0, 0, 1). The characteristics of the hidden order are discussed based
on the local 5f^2 electron picture.
