A hollow golf club head comprises: a face portion; a crown portion; a sole portion; and a side portion between the crown portion and sole portion, extending from a toe-side edge to a heel-side edge of the club face through the back face of the head. The crown portion is provided with a thickness-reduced part having a thickness of from 0.4 to 0.7 mm. The side portion is provided with: at least two vertical buttressed parts each extending from the crown portion to the sole portion and having a thickness of more than 0.6 mm but not more than 2.0 mm; and a thin part formed between the vertical buttressed parts and having a thickness of not more than 0.6 mm.