Multilayer film

A multi-layer film having a layer structure that a polyvalent metal compound-containing layer (B) adjoins one side or both sides of a polymer layer (A), the polymer layer (A) containing a carboxyl group-containing polymer as one and only one polymer element making up the polymer layer (A), and the carboxyl group-containing polymer containing a polyvalent metal salt, wherein

• (1) the multi-layer film has either a layer structure (i) of the polymer layer (A)/the polyvalent metal compound-containing layer (B), and the polymer layer (A) has a concentration gradient structure that the concentration of the polyvalent metal salt of the carboxyl group-containing polymer in the polymer layer (A) is continuously reduced up to a surface opposite to a surface adjoining the polyvalent metal compound-containing layer (B) of the polymer layer (A) in a thickness-wise direction from the adjoining surface, or
• a layer structure (ii) of the polyvalent metal compound-containing layer (B)/the polymer layer (A)/the polyvalent metal compound-containing layer (B), and the polymer layer (A) has a concentration gradient structure that the concentration of the polyvalent metal salt of the carboxyl group-containing polymer in the polymer layer (A) is continuously reduced up to a central portion of the polymer layer (A) in a thickness-wise direction from two surfaces each adjoining to one of the polyvalent metal compound-containing layers (B), and
• (2) a peak ratio A1560/A1700 of the height A1560 of an absorption peak at a wave number of 1560 cm−1 to the height A1700 of an absorption peak at a wave number of 1700 cm−1 as determined on the basis of an infrared absorption spectrum of the polymer layer (A) is at least 0.25.