An electric system stabilizing system for aircraft, comprising at least: an electric system including a DC power supply (13L, 13R) and an AC power supply (14L, 14R, 124, 171) as an electric power supply device, and an AC power supply bus (21L, 21R) connected to the AC power supply (14L, 14R, 124, 171), the electric system being configured to supply electric power to at least one electric device connected to the AC power supply bus (21L, 21R) mounted in the aircraft, wherein regenerative power is generated in at least the electric device connected to the AC power supply bus (21L, 21R); and a power stabilizing device (30L, 30R)connected to the AC power supply bus (21L, 21R) and to the DC power supply (13L, 13R), to stabilize an output of the electric power supply device; wherein the DC power supply (13L, 13R) is configured to absorb regenerative power from the electric device and transiently supply electric power to the electric device (15, 16); wherein the power stabilizing device (30L, 30R) includes: a power converter section including a PWM converter; and a power stabilizing control section for controlling the power converter section; and wherein the power stabilizing control section monitors at least one of a change in a voltage and a change in a frequency in the AC power supply bus (21L,21R), and controls charging/discharging of the DC power supply (13L,13R) according to a result of the monitoring, to stabilize the electric system. an electric system including a DC power supply (13L, 13R) and an AC power supply (14L, 14R, 124, 171) as an electric power supply device, and an AC power supply bus (21L, 21R) connected to the AC power supply (14L, 14R, 124, 171), the electric system being configured to supply electric power to at least one electric device connected to the AC power supply bus (21L, 21R) mounted in the aircraft, wherein regenerative power is generated in at least the electric device connected to the AC power supply bus (21L, 21R); and a power stabilizing device (30L, 30R)connected to the AC power supply bus (21L, 21R) and to the DC power supply (13L, 13R), to stabilize an output of the electric power supply device; wherein the DC power supply (13L, 13R) is configured to absorb regenerative power from the electric device and transiently supply electric power to the electric device (15, 16); wherein the power stabilizing device (30L, 30R) includes: a power converter section including a PWM converter; and a power stabilizing control section for controlling the power converter section; and wherein the power stabilizing control section monitors at least one of a change in a voltage and a change in a frequency in the AC power supply bus (21L,21R), and controls charging/discharging of the DC power supply (13L,13R) according to a result of the monitoring, to stabilize the electric system. a power converter section including a PWM converter; and a power stabilizing control section for controlling the power converter section; and wherein the power stabilizing control section monitors at least one of a change in a voltage and a change in a frequency in the AC power supply bus (21L,21R), and controls charging/discharging of the DC power supply (13L,13R) according to a result of the monitoring, to stabilize the electric system.