Biaxially stretched polypropylene film having high surface elastic modulus

PROBLEM TO BE SOLVED: To enhance the surface elastic modulus of a biapially stretched polypropylene film by specifying the average isotactic block length of polypropylene molecules, n-heptane insoluble and soluble fractions, a ratio of a wt. average mol.wt. to a number average mol.wt. in mol.wt. distribution, and the surface elastic moduli of a base layer in longitudinal and lateral directions. SOLUTION: In polypropylene of a biaxially stretched polypropylene film, the average isotactic block length of polypropylene molecules between two structural carbon chain flaws exceeds 40 on an average and an n-heptane insoluble fraction measured by a C-NMR spectral method has a carbon chain isotactic index of at least 95%, a ratio of a wt. average mol.wt. Mw to a number average mol.wt. Mn in mol.wt. distribution is below 4 and an n-heptane soluble fraction is below 1%. The elastic modulus of a base layer is made larger than 2400N/mm in the longitudinal direction of the base layer and 4800N/mm in the lateral direction thereof and the film may be composed of a multilayer or a single layer.