A tensioning gear the width dimension of which can be suppressed to be small and a power supply system provided with the tensioning gear are provided. The tensioning gear (3) includes a main body (31), a tension generator, a pair of first terminals (33, 34) and a pair of second terminals (35, 36). The main body (31) has a surface (311) and a back surface at two sides at the front-rear direction. One end at the left-right direction of the tension generator is provided with a fixed part fixed relative to the main body (31). A storage recess (313) for accommodating the pair of first terminals (33, 34) and the pair of second terminals (35, 36) is formed on the surface (311). A holding recess for accommodating the fixed part is formed on the back surface.