An endoscope system comprising: a light source device; an endoscope configured to irradiate a subject with measurement light; and a processor device that includes a processor and is configured to generate a subject image related to the subject, wherein the processor is configured to perform at least one of a first control that prohibits switching of a mode to a length measurement mode in which a measurement image in which a measurement marker used to measure a size of the subject is displayed is displayed on a display in a case where an operation for switching a mode to the length measurement mode is performed and a setting condition related to the light source device, the endoscope, or the processor device corresponds to a predetermined prohibition setting condition; a second control that disables a setting change operation for the setting condition in a case where the setting change operation is performed in the length measurement mode and the setting condition to be changed to by the setting change operation corresponds to the prohibition setting condition; or a third control that switches a mode to another mode from the length measurement mode in a case where the setting change operation for the setting condition is performed in the length measurement mode and the setting condition to be changed to by the setting change operation corresponds to the prohibition setting condition; wherein the endoscope includes a measurement light-emitting unit that emits the measurement light and an image pickup optical system that is provided in a distal end part at a position different from a position of the measurement light-emitting unit and receives light from the subject, and the measurement light is emitted obliquely with respect to an optical axis of the image pickup optical system, and the processor is configured to acquire the subject image that includes a measurement light-irradiation region appearing on the subject due to irradiation with the measurement light, detect a position of the measurement light-irradiation region from the subject image, and display the measurement image on a display, wherein the measurement marker is set according to the position of the measurement light-irradiation region. a light source device; an endoscope configured to irradiate a subject with measurement light; and a processor device that includes a processor and is configured to generate a subject image related to the subject, wherein the processor is configured to perform at least one of a first control that prohibits switching of a mode to a length measurement mode in which a measurement image in which a measurement marker used to measure a size of the subject is displayed is displayed on a display in a case where an operation for switching a mode to the length measurement mode is performed and a setting condition related to the light source device, the endoscope, or the processor device corresponds to a predetermined prohibition setting condition; a second control that disables a setting change operation for the setting condition in a case where the setting change operation is performed in the length measurement mode and the setting condition to be changed to by the setting change operation corresponds to the prohibition setting condition; or a third control that switches a mode to another mode from the length measurement mode in a case where the setting change operation for the setting condition is performed in the length measurement mode and the setting condition to be changed to by the setting change operation corresponds to the prohibition setting condition; wherein the endoscope includes a measurement light-emitting unit that emits the measurement light and an image pickup optical system that is provided in a distal end part at a position different from a position of the measurement light-emitting unit and receives light from the subject, and the measurement light is emitted obliquely with respect to an optical axis of the image pickup optical system, and a first control that prohibits switching of a mode to a length measurement mode in which a measurement image in which a measurement marker used to measure a size of the subject is displayed is displayed on a display in a case where an operation for switching a mode to the length measurement mode is performed and a setting condition related to the light source device, the endoscope, or the processor device corresponds to a predetermined prohibition setting condition; a second control that disables a setting change operation for the setting condition in a case where the setting change operation is performed in the length measurement mode and the setting condition to be changed to by the setting change operation corresponds to the prohibition setting condition; or a third control that switches a mode to another mode from the length measurement mode in a case where the setting change operation for the setting condition is performed in the length measurement mode and the setting condition to be changed to by the setting change operation corresponds to the prohibition setting condition; the processor is configured to acquire the subject image that includes a measurement light-irradiation region appearing on the subject due to irradiation with the measurement light, detect a position of the measurement light-irradiation region from the subject image, and display the measurement image on a display, wherein the measurement marker is set according to the position of the measurement light-irradiation region.