There are provided an ink jet head unit which is capable of incorporating a large number of ink jet heads with ease and precision, and an ink jet printing apparatus incorporating the ink jet head unit. The ink jet head unit performs printing color printing by using a plurality of ink nozzle arrays each for use in printing one line, the plurality of ink nozzle arrays corresponding to a plurality of basic colors. A plurality of head groups are each formed by a plurality of ink jet heads, and have the plurality of ink nozzle arrays arranged therein such that the plurality of ink nozzle arrays are divided among the plurality of head groups. A plurality of sub-carriages have respective ones of the plurality of head groups mounted thereon. A unitizing carriage has the plurality of sub-carriages mounted thereon.