The growth mechanism of a single gas bubble in a highly viscous volatile-saturated magmatic melt during its rapid decompression is investigated. A new analytical solution of the problem is shown. It correctly describes the bubble growth dynamics in a wide range of supersaturations achieved as a result of decompression and at all stages of the process including the transitional one. Accounting for this stage is extremely important if we consider highly-viscous magmatic melts. It is shown that after some time bubble growth is determined solely by diffusion, and the process has a self-similar character.