A magma-discharge stepdiagram, which shows a relationship between time and cumulative discharge volume/mass of a volcano, is a useful tool for estimating a magma supply/discharge system beneath the volcano. We propose a model to explain the shape of the magma-discharge stepdiagram, and estimate effects of an abrupt change in crustal stress/strain field, which can modulate the shape of the stepdiagram. Using the shape of the stepdiagram, volcanoes can be classified into four types: time-predictable (TP), volume-predictable (VP), strictly-periodic (SP), and nonpredictable (NP) volcanoes. In the TP volcano, the length of a dormant period is proportional to the magma-discharge volume of the preceded eruption. In the VP volcano, the discharge volume of an eruption is proportional to the length of the preceded dormancy. In the SP volcano, both a length of a dormant period and a discharge volume of an eruption do not change. In the NP volcano, both a length of a dormant period and a discharge volume of an eruption do not have clear predictability. The predictability in the TP, VP, and SP volcanoes can be explained using a simple magma supply/discharge model beneath a volcano, assuming that ascent pressure at the top of a magma reservoir (Pm) monotonously increases in a dormant period, and that magma-discharge volume is proportional to the decrease of Pm at an eruption. Besides these two assumptions, the TP and VP volcanoes need a constant upper limit (Pu) and a constant lower limit (Pl) of Pm, respectively. The SP volcano needs constant Pu and Pl. The magma-discharge stepdiagrams of the TP, VP, and SP volcanoes can be modified by a large intrusive event, changes of physico-chemical parameters in a magma reservoir, magma-supply rate from lower crust or mantle, structural evolution of a magma-supply/discharge system, a small eruption which does not affect the predictability of a magma discharge stepdiagram, and an abrupt change of crustal stress/strain field. Such modifications spoil the predictability in the stepdiagrams and thus make the NP volcano. An abrupt change of crustal stress/strain field causes a change of Pm and/or Pu and thus can modify a magma-discharge stepdiagram. Particularly, an increase of Pm and/or a decrease of Pu can trigger an eruption. Upward shift of a magma-discharge stepdiagram may be a key to finding such trigger of an eruption and to estimating mechanism of triggering.