The paper contains a theory of laser action in a so called SPASER, a laser with a surface plasmon. The goal of the theory development is an ascertainment of physical nature of SPASER lasers that were successfully put into action by M. A. Noginov et al, announced in the paper entitled Demonstration of a spaser-based nanolaser, and by Xiang Zhang et al, the paper Plasmon lasers at deep subwavelength scale, both papers in Nature Advance online publication, August 2009. Plasmonic modes have huge losses but simultaneously the probabilities of radiative processes in atoms (molecules) with emission of radiation into such modes are increased thousandfold as compared with probabilities to radiate in free space. This makes clear how the losses are compensated by amplification. It is made apparent the kinship of physical nature of the phenomenon with that of of the SERS.