Among the p-type filled skutterudites, Ce single-filled (Co,Fe)Sb₃ is known as a promising thermoelectric material. Here, we try to enhance the thermoelectric properties of Ce-filled (Co,Fe)Sb₃ by co-filling of Ga. We synthesize the samples in the nominal compositions Ga_xCeFe_3.5Co_0.5Sb_12−x/3 (x = 0.06, 0.09, 0.15, and 0.21) and examined their thermoelectric properties from room temperature to 773 K. It is confirmed that Ga can occupy not only the void site but also the Sb site, i.e., the chemical formula can be expressed as (Ga_VF)_2x/3CeFe_3.5Co_0.5Sb_12−x/3(Ga_Sb)_x/3, where Ga_VF and Ga_Sb mean Ga in the void and Sb sites, respectively. Ga contributes to optimize the carrier concentration as well as to reduce the lattice thermal conductivity. Owing to these Ga contributions, the material’s thermoelectric figure of merit zT is enhanced and reaches 0.85 in maximum at 773 K, which is obtained for the sample with x = 0.15.