An ink-jet printing apparatus comprising: a web roll heating/supporting mechanism for supporting and heating a web roll formed by winding a sheet-like resin printing medium into a roll shape; a plurality of ink-jet printing heads (12K, 12C, 12M, 12Y, 12W) which eject a plurality of water-based inks onto the printing medium that is wound off from the web roll and transported in a feeding direction thereof; a plurality of under heaters which each heat the printing medium from a rear side surface of the printing medium opposed to a front side surface thereof which faces to the respective ink-jet printing heads; a take-up mechanism for winding the printing medium around a take-up roll;; a tension mechanism for applying a tension force to a portion of the printing medium which is located between the web roll heating/supporting mechanism and the take-up mechanism such that the portion of the printing medium is maintained in a uniform state without any deflection in the feeding direction; a plurality of temperature measuring devices which each measure a temperature of a surface of the printing medium; and a heating amount control device for controlling a heating amount of the web roll heating/supporting mechanism and heating amounts of the plurality of under heaters, in which when setting a plurality of measuring positions of the printing medium comprising a position of the printing medium at which the printing medium is wound into the web roll in the web roll heating/supporting mechanism and positions of the printing medium which are respectively located just below the plurality of ink-jet printing heads, the plurality of temperature measuring devices measure the temperatures of the surface of the printing medium at the plurality of measuring positions, respectively; and the heating amount control device controls the heating amount of the web roll heating/supporting mechanism and the heating amounts of the plurality of under heaters on the basis of the temperatures measured by the plurality of temperature measuring devices such that a difference between the temperatures measured at the measuring positions in the portion of the printing medium to which a tension force is applied falls within the range of 10°C. a web roll heating/supporting mechanism for supporting and heating a web roll formed by winding a sheet-like resin printing medium into a roll shape; a plurality of ink-jet printing heads (12K, 12C, 12M, 12Y, 12W) which eject a plurality of water-based inks onto the printing medium that is wound off from the web roll and transported in a feeding direction thereof; a plurality of under heaters which each heat the printing medium from a rear side surface of the printing medium opposed to a front side surface thereof which faces to the respective ink-jet printing heads; a take-up mechanism for winding the printing medium around a take-up roll;; a tension mechanism for applying a tension force to a portion of the printing medium which is located between the web roll heating/supporting mechanism and the take-up mechanism such that the portion of the printing medium is maintained in a uniform state without any deflection in the feeding direction; a plurality of temperature measuring devices which each measure a temperature of a surface of the printing medium; and a heating amount control device for controlling a heating amount of the web roll heating/supporting mechanism and heating amounts of the plurality of under heaters, in which when setting a plurality of measuring positions of the printing medium comprising a position of the printing medium at which the printing medium is wound into the web roll in the web roll heating/supporting mechanism and positions of the printing medium which are respectively located just below the plurality of ink-jet printing heads, the plurality of temperature measuring devices measure the temperatures of the surface of the printing medium at the plurality of measuring positions, respectively; and the heating amount control device controls the heating amount of the web roll heating/supporting mechanism and the heating amounts of the plurality of under heaters on the basis of the temperatures measured by the plurality of temperature measuring devices such that a difference between the temperatures measured at the measuring positions in the portion of the printing medium to which a tension force is applied falls within the range of 10°C.