We intend to perform the magnetic field penetration depth \lambda measurements on theoxygen reduced and Nb-doped SrTiO_3 -- the first oxide superconductors where two-gap superconductivity was detected an almost 30 years ago. The main interest to study SrTiO_3 in comparison with the well-known double-gap MgB_2 is that in SrTiO_3 the one-gap, the two-gap, the transition from the one- to the two-gap superconductivity, as well as, the two-gapsuperconductivity with the various gap ratios \Delta_1/\Delta_2 can be studied by simply changing the doping level. Our preliminary experiments on one of the original samples studied by Binning et al., [PRL 45, 1352 (1980)] suggest that even though \lambda in SrTiO_3 is of the order of 2-2.5 micron, the muon experiments are feasible. Key words: penetration depth, two-gap superconductivity, Nb doped and oxygen reduced SrTiO_3.