Magma effusion and tremor sometimes occur in quite regular intervals with periodic pressure changes, as observed in volcanic eruptions at Kilauea Volcano, lava dome growth at Unzen Volcano, and tremor episodes at Izu-Oshima Volcano. These events have more or less asymmetric pressure change of quasi-sinusoidal to saw-toothed shape with longer time elapsing in build-up of the fluid pressure than in its relaxation. Such cyclic fluid effusion can be explained systematically by a simple model in which fluid is supplied at a constant rate, accumulates in an elastic chamber, and flows out in a cylindrical vent that may widen or narrow due to viscous deformation of the surrounding country rock. ¿ American Geophysical Union 1996