  The results on ultra-high-energy cosmic rays (UHECR) mass composition
obtained with the Telescope Array surface detector are presented. The analysis
employs the boosted decision tree (BDT) multivariate analysis built upon 14
observables related to both the properties of the shower front and the lateral
distribution function. The multivariate classifier is trained with Monte-Carlo
sets of events induced by the primary protons and iron. An average atomic mass
of UHECR is presented for energies $10^{18.0}-10^{20.0}\ \mbox{eV}$. The
average atomic mass of primary particles shows no significant energy dependence
and corresponds to $\langle \ln A \rangle = 2.0 \pm 0.1 (stat.) \pm 0.44
(syst.)$. The result is compared to the mass composition obtained by the
Telescope Array with $\mbox{X}_{\mbox{max}}$ technique along with the results
of other experiments. Possible systematic errors of the method are discussed.
