A thin film magnetic head of which the insulation layer is made of a polyimide resin obtained by heat-curing a polyimide precursor represented by the following general formula (I) or (II) or a mixture thereof not only has excellent characteristics with respect to the insulation layer, i.e. satisfactory flatness, high glass transition temperature and excellent adhesiveness but also is free from film defects, i.e., exhibits high reliability: ##STR1## wherein R is, e.g., ##STR2## Ar.sup.1 is, e.g., ##STR3## Ar.sup.2 is, e.g. ##STR4## n is an integer of 1 to 100, and m is an integer of 10 to 500.