A puzzling behavior of transition phase in the resonant tunneling through a quantum dot can be understood in a natural way via a formation of electron molecule in the quantum dot. In this case the electron transport proceeds through a the quasi-stationary doorway state, which emerges when the number of electrons occupying the dot reaches a certain "critical" value. Our estimation of this quantity agrees with the experimental data. The dependence of the critical number on the dot's size is predicted as well.