A power system stabilizing device includes an arithmetic unit that constructs a system model by collecting various kinds of system information, calculates a transient stability under an assumed accident condition beforehand, and selects, for each assumed accident in advance, a target solar power generator for shutdown and a target power generator to be disconnected to stabilize a power system, and a control unit that shutdowns the shutdown target solar power generator and disconnects the disconnection target power generator from the power system which are selected by the arithmetic unit when a system accident occurs.