A displacement detecting device includes a main scale and a detecting head unit. The detecting head unit includes a light source, a light receiving unit, and an index scale group that is disposed in the middle of a light path from the main scale to the light receiving unit. The index scale group includes two or more index scales including diffraction gratings, respectively. A positive s-th order diffracted light and a negative s-th order diffracted light of diffracted lights from the main scale are used as the signal lights. The displacement detecting device satisfies a first condition and a second condition, the first condition being expressed as follows: λ×(u1−u2)×(mN÷g)=2×sin α, the second condition being expressed as follows: λ×u1×(mN÷g)−sin α≠λ÷g×Σi=1N (ti×mi).