NOVELTY - A polylactic acid resin composition has difference in melting point after repeating temperature rise and fall three times at 25-260 degrees C at a rate of 20 degrees C/minute in nitrogen atmosphere, of 15 degrees C or less, reduced viscosity decreasing rate of less than 30% after maintaining the composition for 100 hours at 80 degrees C and 95% relative humidity, and stereo complex crystallinity degree of 80% or more. The stereo complex crystallinity degree is calculated from the crystal melting enthalpy of stereo complex phase polylactic acid and homo phase polylactic acid.    USE - Polylactic acid resin composition used for forming polylactic acid unstretched film and stretched film (both claimed). Uses include but are not limited to liquid crystal display, plasma display, organic electroluminescent display, polarizing plate protective film for field emission display, optical film, solar cell back surface protection film, film for printer ribbon, film for thermal stencil printing, antireflection film, water vapor permeation prevention film, etc.    ADVANTAGE - The polylactic acid resin composition forms film having excellent heat resistance and heat-and-moisture resistance.    DETAILED DESCRIPTION - A polylactic acid resin composition has difference in melting point after repeating temperature rise and fall three times at 25-260 degrees C at a rate of 20 degrees C/minute in nitrogen atmosphere, of 15 degrees C or less, reduced viscosity decreasing rate of less than 30% after maintaining the composition for 100 hours at 80 degrees C and 95% relative humidity, and stereo complex crystallinity degree of 80% or more. The stereo complex crystallinity degree is calculated using the formula: S(%)=(( Delta )Hms/( Delta )Hms+( Delta )Hmh)x 100%, where ( Delta )Hms is the crystal melting enthalpy (J/g) of stereo complex phase polylactic acid and ( Delta )Hmh is the crystal melting enthalpy (J/g) of homo phase polylactic acid. The composition shows crystal melting peak at 190 degrees C or more corresponding to the melting temperature of the stereo complex phase polylactic acid and crystal melting peak at 190 degrees C or less corresponding to the melting temperature of the homo phase polylactic acid, in the differential scanning calorimetry measurement. INDEPENDENT CLAIMS are included for the following:    (1) polylactic acid type unstretched film, which contains the polylactic acid type resin composition; and    (2) polylactic acid type stretched film, which is formed stretching the unstretched film and carrying out heat-setting at temperature ranging between glass transition temperature+30 degrees C and melting point -10 degrees C.