PROBLEM TO BE SOLVED: To provide an elastic composite sheet suitable for the production of a disposable diaper having elastic elongation in the girth direction. SOLUTION: This elastic composite sheet 11 usable as a back sheet 3 of the disposable diaper is formed by intermittently attaching the second sheets 22 inelastically extensible in the girth direction to at least one surface of the first sheet 21 elastically extensible in the girth direction. The second sheet 22 is an assembly of filaments, and has a uniform weight. The apparent density of the second sheet 22 in the direction orthogonal to the girth direction is changed.
