PURPOSE: To enable detecting a step-out quickly and accurately in a power system stabilizer.

CONSTITUTION: A stabilizer comprises a measuring section 4 to measure the quantities of electricity such as current and line voltage at the end of a generator in a power system, variation calculating section 5 to estimate the phase angle of an internal voltage on the basis of the measured quantities of electricity, and to calculate variations of a phase angle from a steady state, speeds from the variations in phase angle with time, and accelerations from the variations in speed with time, and parameter estimating section 6 to estimate parameters including a damping coefficient and synchronization coefficient of the generator model on the basis of detected active power.