Method of forming thin film

A method of forming a thin film comprising vinylidene fluoride homopolymer which comprises the following steps (i) and (ii):

• (i) a step for preparing a green powder product of vinylidene fluoride homopolymer comprising crystal form I alone or as main component, by subjecting vinylidene fluoride to radical polymerization in the presence of a bromine compound or iodine compound having 1 to 20 carbon atoms which contains at least one moiety represented by the formula (1):
• wherein X1 iodine atom or bromine atom;
• Rf1 and Rf2 are the same or different and each is selected from fluorine atom or perfluoroalkyl groups having 1 to 5 carbon atoms;
• and
• (ii) a step for forming a thin film on a substrate surface by using vinylidene fluoride homopolymer which comprises crystal form I alone or as main component and is obtained from the green powder product of vinylidene fluoride homopolymer comprising crystal form I alone or as main component,
• wherein in the vinylidene fluoride homopolymers comprising crystal form I alone or as main component, when attention is given to proportions of the respective vinylidene fluoride homopolymers having crystal form I, II or III in the green powder product of vinylidene fluoride homopolymer which are calculated by IR analysis, the proportion of vinylidene fluoride homopolymers having crystal form I satisfies both of (Equation 1):
• 100≧I-form/(I-form+II-form)≧50% by weight (Equation 1)
• and (Equation 2):
• 100≧I-form/(I-form+III-form)≧50% by weight (Equation 2).