Figure-eight solutions are solutions to planar equal mass three-body problem under homogeneous or inhomogeneous potentials. They are known to be invariant under the transformation group $D_6$: the dihedral group of regular hexagons. Numerical investigation shows that each figure-eight solution has some bifurcation points. Six bifurcation patterns are known with respect to the symmetry of the bifurcated solution. In this paper we will show the followings. The variational principle of action and group theory show that the bifurcations of every figure-eight solution are determined by the irreducible representations of $D_6$. Each irreducible representation has one to one correspondence to each bifurcation. This explains numerically observed six bifurcation patterns. In general, in Lagrangian mechanics, bifurcations of a periodic solution is determined by irreducible representations of the transformation group that leaves this solution invariant.