A scroll compressor comprising: a fixed scroll and a movable scroll which respectively have a spiral lap (24b, 26b) placed on one surface of a plate (24a, 26a); wherein the lap of the fixed scroll and the lap of the movable scroll being interlocked, whereby a compression chamber is formed between the lap of the fixed scroll and the lap of the movable scroll which are adjacent to each other; and at least one of the laps (24b, 26b) of the fixed scroll or the movable scroll describing a spiral shape in which a base radius of an involute of the lap (24b, 26b) decreases as a winding angle increases in a region extending from a winding start part of the lap (24b, 26b) to a winding middle part thereof; and, in a region extending from the winding middle part of the lap (24b, 26b) to a winding end part thereof, describing a spiral shape in which the base radius of an inner involute of the lap (24b, 26b) decreases and the base radius of an outer involute of the lap (24b, 26b) either increases or stays constant as the winding angle increases; or describing a spiral shape in which the base radius of the inner involute of the lap (24b, 26b) stays constant and the base radius of the outer involute of the lap (24b, 26b) either increases or stays constant as the winding angle increases, wall thickness of the lap (24b, 26b) is expressed as t2 < t3 < t1 or t2 < t1 < t3, wherein t1 is thickness of the winding start part of the lap (24b, 26b), t2 is thickness of the winding middle part of the lap (24b, 26b), and t3 is thickness of the winding end part of the lap (24b, 26b), wherein the winding middle part of the lap (24b, 26b) ranges from an inner middle point (26bm1) positioned a half to one lap-turn away from an outer involute start point of the lap (24b, 26b) toward an outer involute end point of the lap (24b, 26b), to an outer middle point (26bm2) positioned a half to one lap-turn away from the outer involute end point of the lap (24b, 26b) toward the outer involute start point of the lap (24b, 26b), and wherein the lap (24b, 26b) describes a spiral shape in which a winding angle at a compression-chamber-forming point is smaller than a winding angle at an inner involute end point of the lap (24b, 26b), the compression-chamber-forming point being a point where an outermost compression chamber is formed, the point being included in the outer involute of the lap (24b, 26b), and the point nearest to the outer involute end point of the lap (24b, 26b); and the outermost compression chamber being a compression chamber positioned on the outermost of the plate (24a, 26a) in a radial direction. a fixed scroll and a movable scroll which respectively have a spiral lap (24b, 26b) placed on one surface of a plate (24a, 26a); wherein the lap of the fixed scroll and the lap of the movable scroll being interlocked, whereby a compression chamber is formed between the lap of the fixed scroll and the lap of the movable scroll which are adjacent to each other; and at least one of the laps (24b, 26b) of the fixed scroll or the movable scroll describing a spiral shape in which a base radius of an involute of the lap (24b, 26b) decreases as a winding angle increases in a region extending from a winding start part of the lap (24b, 26b) to a winding middle part thereof; and, in a region extending from the winding middle part of the lap (24b, 26b) to a winding end part thereof, describing a spiral shape in which the base radius of an inner involute of the lap (24b, 26b) decreases and the base radius of an outer involute of the lap (24b, 26b) either increases or stays constant as the winding angle increases; or describing a spiral shape in which the base radius of the inner involute of the lap (24b, 26b) stays constant and the base radius of the outer involute of the lap (24b, 26b) either increases or stays constant as the winding angle increases, wall thickness of the lap (24b, 26b) is expressed as t2 < t3 < t1 or t2 < t1 < t3, wherein t1 is thickness of the winding start part of the lap (24b, 26b), t2 is thickness of the winding middle part of the lap (24b, 26b), and t3 is thickness of the winding end part of the lap (24b, 26b), wherein the winding middle part of the lap (24b, 26b) ranges from an inner middle point (26bm1) positioned a half to one lap-turn away from an outer involute start point of the lap (24b, 26b) toward an outer involute end point of the lap (24b, 26b), to an outer middle point (26bm2) positioned a half to one lap-turn away from the outer involute end point of the lap (24b, 26b) toward the outer involute start point of the lap (24b, 26b), and wherein the lap (24b, 26b) describes a spiral shape in which a winding angle at a compression-chamber-forming point is smaller than a winding angle at an inner involute end point of the lap (24b, 26b), the compression-chamber-forming point being a point where an outermost compression chamber is formed, the point being included in the outer involute of the lap (24b, 26b), and the point nearest to the outer involute end point of the lap (24b, 26b); and the outermost compression chamber being a compression chamber positioned on the outermost of the plate (24a, 26a) in a radial direction.