The ascent of magma in a deformable cylindrical vent is analyzed to reveal the character and extent of mechanical disturbances in vent geometry and associated effects on the magma ascent process. Both magma and surrounding country rock are assumed to behave as Newtonian fluids. According to the analysis, any disturbance involving the deformation of conduit is transmitted upward at approximately same speed as magma motion, while the disturbance hardly influences the state below. In particular, irregular vent geometry containing a bump or a constriction propagates upward like a wave, being accompanied by a pressure dipole that is composed of a pair of positive and negative pressure deviations. Since magma flux is strongly controlled by the pressure below, the persistence of high magma pressure in the magma reservoir favors expansion of the vent and can lead to a volcanic eruption. On the contrary, a low magma pressure tends to result in the closure of the conduit above. Therefore if the effective viscosity of the country rock is sufficiently low as might be the case when it is partially molten, a vent can be flexibly opened or closed in response to pressure in the magma reservoir.