A surroundings observation device for use in a working machine, comprising: at least one image capturing device providable on the working machine to capture an image of an object around the working machine and thereby obtain a captured image; an operating part for operating the working machine; a specifying part for specifying a position of the object in the captured image; a first setting part for setting a detection region where the object is to be detected in the captured image; a responsive action part for executing a predetermined responsive action in a case that the object is in the detection region in the captured image, wherein the specifying part encloses the object with a polygonal frame in the captured image; characterized in that the specifying part sets a straight line which passes a point that is on the polygonal frame and is closest to a to-machine side, and extends in a front orthogonal direction, the front orthogonal direction being orthogonal to a forward direction of the working machine in the captured image, and the to-machine side being a side closer to the working machine in the forward direction; sets a first end point which is on the straight line and attributes to a first apex of a plurality of apexes of the polygonal frame that is at a first side on one side in the front orthogonal direction and closer to the to-machine side; sets a second end point which is on the straight line and attributes to a second apex of the plurality of apexes that is at a second side on the other side in the front orthogonal direction and closer to the to-machine side; sets a midpoint between the first end point and the second end point; and determines one of the first end point, the second end point, and the midpoint as a coordinate indicative of the position of the object according to an operation pattern of the operating part. at least one image capturing device providable on the working machine to capture an image of an object around the working machine and thereby obtain a captured image; an operating part for operating the working machine; a specifying part for specifying a position of the object in the captured image; a first setting part for setting a detection region where the object is to be detected in the captured image; a responsive action part for executing a predetermined responsive action in a case that the object is in the detection region in the captured image, wherein the specifying part encloses the object with a polygonal frame in the captured image; characterized in that the specifying part sets a straight line which passes a point that is on the polygonal frame and is closest to a to-machine side, and extends in a front orthogonal direction, the front orthogonal direction being orthogonal to a forward direction of the working machine in the captured image, and the to-machine side being a side closer to the working machine in the forward direction; sets a first end point which is on the straight line and attributes to a first apex of a plurality of apexes of the polygonal frame that is at a first side on one side in the front orthogonal direction and closer to the to-machine side; sets a second end point which is on the straight line and attributes to a second apex of the plurality of apexes that is at a second side on the other side in the front orthogonal direction and closer to the to-machine side; sets a midpoint between the first end point and the second end point; and determines one of the first end point, the second end point, and the midpoint as a coordinate indicative of the position of the object according to an operation pattern of the operating part. sets a straight line which passes a point that is on the polygonal frame and is closest to a to-machine side, and extends in a front orthogonal direction, the front orthogonal direction being orthogonal to a forward direction of the working machine in the captured image, and the to-machine side being a side closer to the working machine in the forward direction; sets a first end point which is on the straight line and attributes to a first apex of a plurality of apexes of the polygonal frame that is at a first side on one side in the front orthogonal direction and closer to the to-machine side; sets a second end point which is on the straight line and attributes to a second apex of the plurality of apexes that is at a second side on the other side in the front orthogonal direction and closer to the to-machine side; sets a midpoint between the first end point and the second end point; and determines one of the first end point, the second end point, and the midpoint as a coordinate indicative of the position of the object according to an operation pattern of the operating part.