1. A surrounding area monitoring device for monitoring an area around a work machine, comprising: a display device; a worker position detection unit that detects a position of a worker working around the work machine; an image capturing unit mounted at the work machine, which obtains a surrounding area image by capturing an image of an area around the work machine; an image conversion unit that converts the surrounding area image of the area around the work machine, obtained by the image capturing unit, to a bird's eye view image of the work machine; an image generation unit that generates a monitor image based upon the bird's eye view image resulting from conversion via the image conversion unit and the position of the worker detected by the worker position detection unit; a display control unit that displays the monitor image at the display device; a decision-making unit that makes a decision as to whether or not the position of the worker is located within a monitoring area set in advance; and a plurality of work machine condition detection units that detect an attitude of the work machine including a work machine traveling speed at which the work machine is traveling or drive conditions of the work machine, wherein: the image generation unit generates a plurality of monitor images which show conditions within monitoring ranges with varying sizes based upon results of the decision made by the decision-making unit or information provided from at least one of the work machine condition detection units, the monitoring ranges being set around the work machine.