A sustained-release composition for oral administration of a drug, comprising the drug, a mixture of sodium alginate and xanthan gum as a carrier for sustained release and a mixture of hydroxypropyl methylcellulose and propylene glycol alginate as a gel hydration accelerator, which is capable of maintaining a constant drug level in blood for 24 hours or more. Due to rapid gel hydration without forming a non-gelated core, the drug release rate follows zero order kinetics and does not significantly vary with the degree of gastrointestinal motility.