A state estimation system includes a first processor, and the first processor executes: a first state variable measurement process of measuring a first state variable of a monitored object; a second state variable estimation repetition process of repeating a second state variable estimation process of inputting a measurement value of the first state variable into a learned model and acquiring an estimate value of a second state variable outputted from the learned model; and an estimation accuracy decline information output process of outputting estimation accuracy decline information when a predetermined estimate value sudden change determination condition is met with respect to a first estimate value of the second state variable acquired in response to input of a first measurement value of the first state variable.