An ink jet printer, for both back-surface printing and front-surface printing, forms an image with aqueous ink on a web-shaped printing base material by any of the back-surface printing and the front-surface printing. An ink jet printing method uses the ink jet printer. The ink jet printer includes: a conveyance mechanism configured to continuously convey the web-shaped printing base material in forward and reverse directions; a single-pass system ink jet head configured to discharge white aqueous ink to a surface of the web-shaped printing base material conveyed by the conveyance mechanism; a single-pass system ink jet head configured to discharge non-white aqueous ink to the surface of the web-shaped printing base material conveyed by the conveyance mechanism; and a white ink drying device with a drying zone for drying the surface of the web-shaped printing base material having the white aqueous ink adhering thereto.