A fuel injector including a fuel supply tube (8), a plurality of premix tubes (2), a support plate (3) which supports the fuel supply tube and the plurality of premix tubes, a substrate which supports downstream end portions of the plurality of premix tubes, an outer wall which is cylindrical, which forms a plenum (P) inside, a partition plate which partitions the plenum (P) into a fuel plenum (PF) and a cooling air plenum (PA), a baffle which partitions the cooling air plenum (PA) into an upstream cooling air plenum (PA1) and a downstream cooling air plenum (PA2) and has a plurality of cooling holes formed therein, and a cooling air supply tube configured to supply cooling air to the upstream cooling air plenum (PA1), in which the end portion on the downstream side of the fuel supply tube opens in the fuel plenum (PF), and a fuel introduction hole.