A plant evaluation apparatus comprising: a first computation unit configured to compute a first estimated value of a plant state quantity by using a steady-state model indicating a mathematical relationship in a steady state between an equipment parameter and the plant state quantity of plant equipment, and to compute the equipment parameter of the plant equipment according to a difference between the first estimated value of the plant state quantity and an actually measured value of the plant state quantity, the computed equipment parameter being preserved as an equipment parameter in a normal state; a first determination unit configured to compute a second estimated value of the plant state quantity by assigning the equipment parameter in the normal state to an equipment parameter of a non-steady-state model by using the non-steady-state model indicating a mathematical relationship in a non-steady state between the equipment parameter and the plant state quantity of the plant equipment, and to determine whether or not an error between the second estimated value of the plant state quantity and the actually measured value of the plant state quantity is equal to or more than a predetermined threshold value; a second computation unit configured to compute a third estimated value of the plant state quantity by using the steady-state model in a case where the first determination unit determines that the error between the second estimated value of the plant state quantity and the actually measured value of the plant state quantity is equal to or more than the threshold value, and to compute an equipment parameter of the plant equipment according to a difference between the third estimated value of the plant state quantity and the actually measured value of the plant state quantity, the computed equipment parameter being preserved as an equipment parameter in an abnormal state; and a second determination unit configured to determine whether or not a difference between the equipment parameter in the normal state and the equipment parameter in the abnormal state is equal to or more than a predetermined value. a first computation unit configured to compute a first estimated value of a plant state quantity by using a steady-state model indicating a mathematical relationship in a steady state between an equipment parameter and the plant state quantity of plant equipment, and to compute the equipment parameter of the plant equipment according to a difference between the first estimated value of the plant state quantity and an actually measured value of the plant state quantity, the computed equipment parameter being preserved as an equipment parameter in a normal state; a first determination unit configured to compute a second estimated value of the plant state quantity by assigning the equipment parameter in the normal state to an equipment parameter of a non-steady-state model by using the non-steady-state model indicating a mathematical relationship in a non-steady state between the equipment parameter and the plant state quantity of the plant equipment, and to determine whether or not an error between the second estimated value of the plant state quantity and the actually measured value of the plant state quantity is equal to or more than a predetermined threshold value; a second computation unit configured to compute a third estimated value of the plant state quantity by using the steady-state model in a case where the first determination unit determines that the error between the second estimated value of the plant state quantity and the actually measured value of the plant state quantity is equal to or more than the threshold value, and to compute an equipment parameter of the plant equipment according to a difference between the third estimated value of the plant state quantity and the actually measured value of the plant state quantity, the computed equipment parameter being preserved as an equipment parameter in an abnormal state; and a second determination unit configured to determine whether or not a difference between the equipment parameter in the normal state and the equipment parameter in the abnormal state is equal to or more than a predetermined value.