A ballast comprising a first input terminal, a second input terminal, a switch circuit, and a plurality of lamp sets is provided. The switch circuit comprises a first switch and a second switch connected with the first one. The switches are connected with the first and second input terminals respectively. The lamp sets are connected in parallel with each other and have an arrangement sequence. Each of the lamp sets is coupled to the first and second switches and comprises a first lamp having a first filament. The filaments are connected in series according to the arrangement order so that at least one junction is formed in the at least one connection point. The first one of the first filaments is coupled to the first switch. The last one of the first filaments is coupled to the second switch. Thereby, the ballast can be implemented by less internal connection terminals and leads.