PROBLEM TO BE SOLVED: To provide a slope excavation device and a construction method capable of constructing even in a narrow point with limitation on both mountain side and valley side, without using a large crane whose cost is high.SOLUTION: A work platform 3 is installed almost parallel to a mountain side and a valley side of a slope 1. A portal crane 5 is arranged on the work platform 3 for traveling, with an excavator 9 suspended from the portal crane 5. The slope 1 is excavated by the excavator 9 which is suspended from the portal crane 5 that travels on the work platform 3 which is almost parallel to the mountain side and the valley side of the slope 1. In short, excavation of the slope 1 by the excavator 9 that is suspended from the portal crane 5 on the work platform 3 which is almost parallel to the mountain side and the valley side of the slope 1 is repeated each time the portal crane 5 travels, on the work platform 3.
