We present a preliminary result of the kaon semileptonic form factor
calculated at the smallest lattice spacing in the PACS10 configurations,
whose physical volumes are more than (10 fm)$^4$ at the physical point.
The configurations were generated using the Iwasaki gauge action and
$N_f=2+1$ stout-smeared nonperturbatively $O(a)$ improved Wilson quark action
at the three lattice spacings, 0.085, 0.063, and 0.041 fm.
The value of $|V_{us}|$ in the continuum limit is estimated from our results
including the preliminary one. We compare our result of $|V_{us}|$
with the previous results and those through the kaon leptonic decay.