A priority function control apparatus for a construction machine, comprising: a variable displacement hydraulic pump and a pilot pump connected to an engine; an optional device and an attachment connected to the hydraulic pump; an attachment operation lever outputting an operation signal that corresponds to an operation amount when an operator performs an operation; an optional device spool installed in a flow path between the hydraulic pump and the optional device and shifted to control a flow direction of hydraulic fluid that is supplied to the optional device in response to signal pressure from an outside; an attachment spool installed in a flow path between the hydraulic pump and the attachment and shifted to control a start, a stop, and a direction change of the attachment in response to the operation of the attachment operation lever; an electrical operation device for the optional device outputting an operation signal in accordance with an operator's operation; an electro proportional reducing valve installed in a signal path between the pilot pump and the optional device spool to output secondary signal pressure that corresponds to the operation signal of the electrical operation device for the optional device; a hydraulic pump pressure detection sensor installed in an upstream discharge flow path of the hydraulic pump to detect in real time load pressure of the hydraulic pump and to output a detection signal; an attachment pressure detection sensor installed in a flow path between the attachment spool and the attachment to detect in real time load pressure of the attachment and to output a detection signal; and a controller, in which a priority value or a ratio is preset by the operator in consideration of load pressure of the optional device, the controller limiting the signal pressure that is applied from the electro proportional reducing valve to the optional device spool by the operation of the electrical operation device for the optional device to a preset signal pressure so that a difference value between the load pressure of the hydraulic pump that is formed by the load pressure of the attachment when the optional device is not used and the load pressure of the hydraulic pump that is formed by the load pressure of the attachment when the optional device is used becomes equal to or smaller than a preset value by the priority value or the ratio set by the operator, wherein the controller comprises an optional device pressure detection sensor installed in a flow path between the optional device spool and the optional device. a variable displacement hydraulic pump and a pilot pump connected to an engine; an optional device and an attachment connected to the hydraulic pump; an attachment operation lever outputting an operation signal that corresponds to an operation amount when an operator performs an operation; an optional device spool installed in a flow path between the hydraulic pump and the optional device and shifted to control a flow direction of hydraulic fluid that is supplied to the optional device in response to signal pressure from an outside; an attachment spool installed in a flow path between the hydraulic pump and the attachment and shifted to control a start, a stop, and a direction change of the attachment in response to the operation of the attachment operation lever; an electrical operation device for the optional device outputting an operation signal in accordance with an operator's operation; an electro proportional reducing valve installed in a signal path between the pilot pump and the optional device spool to output secondary signal pressure that corresponds to the operation signal of the electrical operation device for the optional device; a hydraulic pump pressure detection sensor installed in an upstream discharge flow path of the hydraulic pump to detect in real time load pressure of the hydraulic pump and to output a detection signal; an attachment pressure detection sensor installed in a flow path between the attachment spool and the attachment to detect in real time load pressure of the attachment and to output a detection signal; and a controller, in which a priority value or a ratio is preset by the operator in consideration of load pressure of the optional device, the controller limiting the signal pressure that is applied from the electro proportional reducing valve to the optional device spool by the operation of the electrical operation device for the optional device to a preset signal pressure so that a difference value between the load pressure of the hydraulic pump that is formed by the load pressure of the attachment when the optional device is not used and the load pressure of the hydraulic pump that is formed by the load pressure of the attachment when the optional device is used becomes equal to or smaller than a preset value by the priority value or the ratio set by the operator, wherein the controller comprises an optional device pressure detection sensor installed in a flow path between the optional device spool and the optional device.