To provide a styrenic resin foamed board which excels in heat insulating properties and flame retardance and is suitably used as a constructional heat insulating material and the like.

The styrenic resin foamed board is obtained by extrusion foaming from an extruder, and the cells are mainly constituted of small diameter cells having a cell diameter in the thickness direction of ≤0.10 mm and large diameter cells having a cell diameter in the thickness direction of 0.15 mm to less than 0.30 mm, and has a ratio of the small diameter cells and the large diameter cells in the total area is ≥85% and, simultaneously, a ratio of the small diameter cells in the total area to the small diameter cells and the large diameter cells in the total area of 30-80% in the cross-sectional area cut in the thickness direction crossing in right angles to the extrusion direction, and furthermore has an amount of butane present in the foamed board 30 days after extrusion foaming is 1.5 wt.% to less than 3.0 wt.%.

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