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 use purpose of a user, the monitoring device comprising: a processor; a memory that stores instructions; a use purpose acquirer that obtains the use purpose of the user according to an input operation performed by the user; a moving image processor that generates the mask processing image by performing a mask processing of changing the entirety or a part of the image area of the movable body of the video into the mask image; a moving image outputter that selectively outputs the video and the mask processing image; and a controller that controls the moving image processor and the moving image outputter according to the use purpose of the user, and outputs one of the original video and the mask processing image, wherein the use purpose acquirer, the moving image processor, the moving image outputter, and the controller correspond to the instructions which are stored in the memory and performed by the processor, and wherein the controller causes the moving image processor to start a prescribed preprocessing according to a prescribed start event generated in advance of the use purpose input operation, which is performed by the user.