Pattern formation of a step on a growing crystal surface induced by a straight line source of atoms, which is escaping from the step at a velocity V_p, is studied with the use of a phase field model. From a straight step, fluctuations of the most unstable wavelength λ_max grow. Competition of intrusions leads to coarsening of the pattern, and survived intrusions grow exponentially. With sufficient strength of the crystal anisotropy, a regular comblike pattern appears. This peculiar step pattern is similar to that observed on a Ga-deposited Si(111) surface. The final period of the intrusions, Λ, is determined when the exponential growth ends. The period depends on the strength Fu of a current noise in diffusion as Λ∼λ_max| ln F_u|: such a logarithmic dependence is confirmed for the first time. A nonmonotonic V_p dependence of Λ indicates that the comblike pattern with a small V_p is related to an unstable growth mode of the free needle growth in a channel. The pattern is stabilized by the guiding linear source.