An ink jet printer for both front-surface printing and back-surface printing, which is configured to perform image formation by discharging aqueous ink to a web-shaped printing base material by a single-pass system, comprising: 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 (16b-16e) 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, which has a carry-in port for conveying the web-shaped printing base material having the white aqueous ink adhering thereto to the white ink drying device and a carry-out port for conveying the web-shaped printing base material having the white aqueous ink adhering thereto out from the white ink drying device, and has a drying zone for drying the surface of the web-shaped printing base material having the white aqueous ink adhering thereto, wherein, in the case of back-surface printing, the white aqueous ink is dried by causing the web-shaped printing base material, which has the white aqueous ink adhering thereto and has been conveyed in the forward direction, to pass through the white ink drying device, and wherein, in the case of front-surface printing, the white aqueous ink is dried by causing the web-shaped printing base material, which has the white aqueous ink adhering thereto and has been conveyed in the reverse direction, to pass through the white ink drying device. 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 (16b-16e) 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, which has a carry-in port for conveying the web-shaped printing base material having the white aqueous ink adhering thereto to the white ink drying device and a carry-out port for conveying the web-shaped printing base material having the white aqueous ink adhering thereto out from the white ink drying device, and has a drying zone for drying the surface of the web-shaped printing base material having the white aqueous ink adhering thereto, wherein, in the case of back-surface printing, the white aqueous ink is dried by causing the web-shaped printing base material, which has the white aqueous ink adhering thereto and has been conveyed in the forward direction, to pass through the white ink drying device, and wherein, in the case of front-surface printing, the white aqueous ink is dried by causing the web-shaped printing base material, which has the white aqueous ink adhering thereto and has been conveyed in the reverse direction, to pass through the white ink drying device.