Disclosed are a method for producing polyhydroxyalkanoates (PHAs) using a halobacterium belonging to the genus Halomonas, wherein the halobacterium can grow in a medium consisting of an inorganic salt and a single organic carbon source and having a pH of 8.8 to 11, and produce PHAs in an amount of 20 wt. % or more based on the dry cell weight, and the halobacterium is cultured in an alkaline medium containing an inorganic salt and one or more organic carbon sources to produce PHAs in an amount of 20 wt. % or more based on the dry cell weight; and the halobacterium belonging to the genus Halomonas, which can grow in a medium consisting an inorganic salt and a single organic carbon source and having a pH of 8.8 to 11, and produce PHAs in an amount of 20 wt. % or more based on the dry cell weight.