An operator of a nuclear plant is provided with an action margin time up to the execution of a next action to be performed in response to a detected accident based on sensor signals received from sensors for the plant. The operator is provided with a displayed graph showing the received sensor signals, estimated sensor signals for the future, the action margin time and an action threshold value corresponding to the next action. Accordingly, the operator can readily grasp how much time is available to perform the next action before the received sensor signals are predicted to exceed the action threshold value.