1. A numerical control unit that controls a machine tool based on a machining program in which operation modes and operation commands to control a servo motor, a spindle motor, and a peripheral device constituting the machine tool are instructed, the numerical control unit comprising: a machining program analysis unit configured to analyze the machining program; an operation-mode-control execution unit configured to control the servo motor, the spindle motor, and the peripheral device based on the operation modes and the operation commands analyzed by the machining program analysis unit; an operation mode screen correspondence storage unit configured to store in advance the relations between an operation-mode-screen displaying operation conditions of the servo motor, the spindle motor, and the peripheral device prepared for each of the operation modes, and, operation mode; an operation mode screen display unit configured to select the operation-mode-screen to be displayed, based on the operation mode screen relations, as an operation-mode-screen, corresponding to the operation mode currently being executed obtained by the operation mode control execution unit, and display the selected operation-mode-screen in a display; a commanded-operation-mode storage unit configured to store identification information of the operation modes in the order of instructions when the plurality of operation modes is instructed by the machining program; an operation mode command history display unit configured to display the identification information of the operation mode stored in the commanded-operation-mode storage unit; and an operation mode selection unit configured to select the operation mode displayed in the operation mode command history display unit, wherein the operation-mode-screen display unit is further configured to display the operation-mode-screens corresponding to the operation mode selected; and wherein the numerical control unit is capable of automatically displaying an operation-mode-screen in accordance with the operation mode being executed.