A heat source unit (2) is a heat source unit where refrigerant circuit constituent parts are provided inside a casing (40) and where the refrigerant circuit constituent parts are changed or added in accordance with capacity or function. Here, a bottom frame (51) forming a bottom surface of the casing (40) has a first bottom frame (51a), on which are provided first refrigerant circuit constituent parts that are common regardless of capacity or function among the refrigerant circuit constituent parts, and a second bottom frame (51b), on which are provided second refrigerant circuit constituent parts that are changed or added depending on capacity or function among the refrigerant circuit constituent parts.