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 angle of an articulated, extensible front work unit provided at the work machine and one or more drive conditions of the work machine including at least one of a work machine traveling speed and a front work unit front-end position wherein the image generation unit generates a plurality of monitor images which show conditions within monitoring ranges with varying sizes based upon information provided from at least one of the work machine condition detection units if the decision-making unit decides that the position of the worker is located within the monitoring area, the monitoring ranges being set around the work machine, and the plurality of monitor images include an immediate range display image showing conditions in an immediate area set around the work machine and a wide range display image showing conditions in a wide area containing the immediate area also set around the work machine.