  Systematic analysis of the deformations of proton and neutron densities in
even-even C isotopes was done based on the method of antisymmetrized molecular
dynamics. The $E2$ transition strength was discussed in relation to the
deformation. We analyze the $B(E2;2^+_1\to 0^+_1)$ in $^{16}$C, which has been
recently measured to be abnormally small. The results suggest the difference of
the deformations between proton and neutron densities in the neutron-rich C
isotopes. It was found that stable proton structure in C isotopes plays an
important role in the enhancement the neutron skin structure as well as in the
systematics of $B(E2)$ in the neutron-rich C.
