In Japan, after establishing the control reserve market in 2021, a Merit-Order (MO) method based on cost per kWh of each generator will be applied to load frequency control (LFC). The conventional Ramp-Rate (RR) method dispatches LFC signal based on ramp-rate of each LFC generator. The MO method may increase frequency fluctuation compared to RR method, because the aggregated ramp-rate of all LFC generators is not fully utilized, which is the result of the change of LFC signals dispatched to some generators are different from the change of Area Requirement (AR). In order to keep frequency controllability as RR method, we propose an LFC signal dispatching MO method considering the change of AR. The effectiveness of the proposed method is evaluated by using the IEEJ AGC30 model. This study shows that the proposed method is able to reduce frequency fluctuation compared to simplified MO method, and to reduce operating costs compared to RR method.