Van der Pol's oscillator is excited by stationary narrow-band noise. The analysis is based on the concept of quasi-static amplitude and phase value, which is completely opposite to the stochastic or Mark's approximation associated with Fokker-Planck's equation. This approximation actually replaces the system with memory with the system without memory. It was shown that no jump is possible in this system, and the formula for the probability of the appearance of a stable region of response amplitudes was derived.