If a rotating machine has its operating speed above its critical one, it must pass the critical speed when accelerated to operating speed or retarted to a halt. This paper deals with the building up of a vibration of uniformly accelerated or retarded the unbalanced rotor which has linear stiffness and damping. The method of contour integral, by which F. M. Lewis worked analogous problem, is used. The figure shows the maximum amplitude concerning to the damping and acceleration. The experimental formula deduced from the calculated graphs is as follows : Remax=3·78√q e-1·16a, a=q0·379γ0·7, 10<q<100, 0<γ<0·20 where Remax is the maximum amplitude measured by the unit of eccentricity, q=N2/h, N being critical speed and h acceleration, and γ is dimensionless damping. During acceleration the instantaneous speed, at which the vibration amplitude becomes maximum, is higher than the critical speed, and this shift percentage from the critical speed is estimated at 86/√q-34γ.