  We report on our calculation of the kaon semileptonic form factors in Nf=2+1
lattice QCD. Chiral symmetry is exactly preserved by using the overlap quark
action for a straightforward comparison with chiral perturbation theory (ChPT).
We simulate three pion masses down to 290 MeV at a single lattice spacing of
0.11 fm and at a strange quark mass very close to its physical value. The form
factors near zero momentum transfer are precisely calculated by using the
all-to-all propagator and twisted boundary conditions. We compare the
normalizations and slopes of the form factors with ChPT and experiments.
