A blow molding method includes: performing a primary blowing on a resin preform to form an intermediate molded product; and performing a final blowing on the intermediate molded product to form a final molded product, and at least one step of performing the primary blowing and the final blowing includes a thinning step of continuously repeatedly performing, within a predetermined time, steps of: stretching an object by blowing; and contracting stretched parts of the object by exhausting blown air.