The discovery of graphene with Dirac cones at the Fermi energy attracted intense interest in the field of two-dimensional materials. However, in many two-dimensional materials, including graphene, Dirac points are gapped by spin-orbit coupling. Here we consider two-dimensional Dirac semimetals which have Dirac-like band dispersion in the presence of spin-orbit coupling protected by nonsymmorphic lattice symmetry. This is of interest because it opens a richer spectrum of optical properties than other topological materials. We choose the model of nonsymmorphic Dirac semimetal α-bismuthine containing anisotropic Dirac cones. We calculated interband and intra-band linear optical conductivity within the formalism based on the density matrix approach and Kubo formula. We show that electronic state in conductance band supports plasmons with quasi-linear anisotropic dispersion. The difference in the interband absorption spectrum can only be observed for electronic states on the Femi surface and a width equal to the plasmon energy. The results suggest that such Dirac semimetals can be promising material for studying  nonlinear optical properties.