A novel and simple purification method for microbial $poly-{eta}-hydroxybutyrate$ (PHS) was developed. Sodium hydroxide was found to be efficient for digesting cell materials. Initial biomass concentration, NaOH concentation, digestion time, and incubation temperature were optimized. When 40 g/l of biomass was incubated in 0.1 N NaOH at $30^{circ}C$ for 1 h, PHB purity of 88.4% with a weight average molecular weight ($M_w$) of 770,000 and a polydispersity index (PI) of 2.4 was recovered with a yield of 90.8% from the biomass which initially contained PHB of a $M_w$ of 780,000 and a PI of 2.3.