  We study superconducting FeSe (Tc = 9 K) exhibiting the
tetragonal-orthorhombic structural transition (Ts = 90 K) without any
antiferromagnetic ordering, by utilizing angle-resolved photoemission
spectroscopy. In the detwinned orthorhombic state, the energy position of the
dyz orbital band at the Brillouin zone corner is 50 meV higher than that of
dxz, indicating the orbital order similar to NaFeAs and BaFe2As2 families.
Evidence of orbital order also appears in the hole bands at the Brillouin zone
center. Precisely measured temperature dependence using strain-free samples
shows that the onset of the orbital ordering (To) occurs very close to Ts, thus
suggesting that the electronic nematicity above Ts is considerably weaker in
FeSe compared to BaFe2As2 family.
