Abstract: A continuous film sheet having a width in a horizontal direction is unwound from a film roll with its axis being substantially horizontally kept. The direction of extension of the film sheet is changed so as to have its width in a vertical direction before the film sheet is slit along its center line into two continuous strips. The two continuous strips are superposed on each other and heat-sealed in lengthwise and transverse directions to form a plurality of rectangular sections defined by heat-sealed sides. The heat-sealed sheet is then slit and cut into separated envelopes having their respective three heat-sealed edges.