We measure the time-dependent $C!P$ violation parameters in $B^{0} to K^{0}_{S}K^{0}_{S}K^{0}_{S}$ decays using $772 times 10^6$ $B {mkern 1.5muoverline{mkern-1.5mu B mkern-0.1mu}mkern 0.1mu}$ pairs collected at the $Upsilon (4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The obtained mixing-induced and direct $C!P$ asymmetries are $-0.71 pm 0.23~{rm(stat)} pm 0.05~{rm(syst)}$ and $0.12 pm 0.16~{rm(stat)} pm 0.05~{rm(syst)}$, respectively. These values are consistent with the Standard Model predictions. The significance of $C!P$ violation differs from zero by 2.5 standard deviations.