  We present the calculation of the $K_{l3}$ form factors with $N_f = 2 + 1$
nonperturbatively $O(a)$-improved Wilson quark action and Iwasaki gauge action
at the physical point on a large volume of (10.9 fm)$^3$ at one lattice spacing
of $a = 0.085$ fm. We extract the form factors from 3-point functions with
three different time separations between the source and sink operators to
confirm suppression of excited state contributions. The form factors are
calculated in very close to the zero momentum transfer, $q^2 = 0$, thanks to
the large volume, so that stable interpolations to $q^2 = 0$ are carried out.
Using our form factors, we obtain the form factor at $q^2 = 0$, $f_+(0) =
0.9603(16)(^{+14}_{\ -4})(44)(19)(1)$, where the first, second, and fifth
errors are statistical, systematic errors from fit functions and the isospin
breaking effect, respectively. The third and fourth errors denote the finite
lattice spacing effects estimated from the renormalization factor and
contribution beyond the leading order SU(3) chiral perturbation theory (ChPT).
The result of $f_+(0)$ yields the Cabibbo-Kobayashi-Maskawa (CKM) matrix
element, $|V_{us}| = 0.2255(13)(4)$, where the first error comes from our
calculation and the second from the experiment. This value is consistent with
the ones determined from the unitarity of the CKM matrix and the $K_{l2}$ decay
within one standard deviation, while it is slightly larger than recent lattice
calculations by at most 1.5 $\sigma$. Furthermore, we evaluate the shape of the
form factors and the phase space integral from our results. We confirm that
those results are consistent with the experiment, and also $|V_{us}|$
determined with our phase space integral agrees with the one in the above.
