We present preliminary results for nucleon form factors including the axial charge and the isovector electric radius in 2+1 flavor QCD at almost the physical pion mass on a $96^4$ lattice with the lattice spacing of 0.085 fm. The configurations are generated with the stout-smeared $O(a)$-improved Wilson quark action and the Iwasaki gauge action at $\beta$=1.82. A large spatial volume of $(8.1 {\rm fm})^3$ allows us to investigate the form factors at small momentum transfer region. We discuss analyses of the momentum dependence of the form factors with the z-expansion.