The present invention provides a production method of biodegradable plastic in which polylactic acid is directly synthesized from lactic acid without employing the conventional route of polylactic acid synthesis from lactic acid via lactide, and apparatus for use in production thereof. In this way, time required for the synthesis of polylactic acid is strikingly shortened and the production cost is reduced. In the production method of biodegradable plastic, polylactic acid, the main component of biodegradable plastic, is obtained by means of condensation polymerization by dehydration of the raw material, lactic acid, under high temperature and reduced pressure in the presence of a metal catalyst.