1. A monitoring device, which generates a mask processing image, obtained by changing an image area of a movable body of a video in a monitoring area into a mask image, and which outputs one of an original video and the mask processing image according to a purpose of use of a user, the monitoring device comprising: a processor; a controller; a moving image processor; and a memory that stores instructions that, when executed by the processor, cause the processor to perform operations including: obtaining the purpose of use of the user according to a purpose of use input operation performed by the user; generating, by the moving image processor, the mask processing image by performing a mask processing of changing an entirety or a part of the image area of the movable body of the video into the mask image; selectively outputting the video and the mask processing image; and controlling, by the controller, the moving image processor and the selectively outputting according to the purpose of use of the user, and outputting one of the original video and the mask processing image, wherein the controller causes the moving image processor to start a prescribed preprocessing according to a prescribed start event generated in advance of the purpose of use input operation, which is performed by the user, and wherein the purpose of use is chosen from a list generated on a display of the monitoring device.