A start-up control device for a power plant including a steam turbine, comprising: an equipment state quantity acquisition unit acquiring an equipment state quantity of the power plant; characterized in that the start-up control device further comprises: a start-up schedule calculation unit calculating a current start-up schedule from a current point after beginning of start-up to a start-up completion based on the equipment state quantity and a predetermined first constraint condition and calculating a start-up schedule changing plan from the current point to the start-up completion based on the equipment state quantity and a predetermined second constraint condition; an equipment operation amount calculation unit calculating an equipment operation amount of the power plant based on the current start-up schedule; a screen display unit displaying start-up completion estimated times with respect to a case in which the current start-up schedule is executed from the current point to the start-up completion and a case in which the start-up schedule changing plan is executed from the current point to the start-up completion, respectively; and an instruction input unit outputting an instruction for switching from the running start-up schedule to the start-up schedule changing plan to the start-up schedule calculation unit in accordance with an input operation by an operator. an equipment state quantity acquisition unit acquiring an equipment state quantity of the power plant; characterized in that the start-up control device further comprises: a start-up schedule calculation unit calculating a current start-up schedule from a current point after beginning of start-up to a start-up completion based on the equipment state quantity and a predetermined first constraint condition and calculating a start-up schedule changing plan from the current point to the start-up completion based on the equipment state quantity and a predetermined second constraint condition; an equipment operation amount calculation unit calculating an equipment operation amount of the power plant based on the current start-up schedule; a screen display unit displaying start-up completion estimated times with respect to a case in which the current start-up schedule is executed from the current point to the start-up completion and a case in which the start-up schedule changing plan is executed from the current point to the start-up completion, respectively; and an instruction input unit outputting an instruction for switching from the running start-up schedule to the start-up schedule changing plan to the start-up schedule calculation unit in accordance with an input operation by an operator. a start-up schedule calculation unit calculating a current start-up schedule from a current point after beginning of start-up to a start-up completion based on the equipment state quantity and a predetermined first constraint condition and calculating a start-up schedule changing plan from the current point to the start-up completion based on the equipment state quantity and a predetermined second constraint condition; an equipment operation amount calculation unit calculating an equipment operation amount of the power plant based on the current start-up schedule; a screen display unit displaying start-up completion estimated times with respect to a case in which the current start-up schedule is executed from the current point to the start-up completion and a case in which the start-up schedule changing plan is executed from the current point to the start-up completion, respectively; and an instruction input unit outputting an instruction for switching from the running start-up schedule to the start-up schedule changing plan to the start-up schedule calculation unit in accordance with an input operation by an operator.