A single-pole miniature circuit breaker, comprising:-- a casing on which there are two terminals for connection, in input and in output from the circuit breaker, to a phase of a low-voltage electric circuit, said casing having a front wall from which an actuation lever protrudes, a rear wall for coupling to a supporting element, two side walls that are substantially mutually parallel and whose distance is equal to a base module M, an upper wall and a lower wall, inside the casing there being provided:--fixed and moving contact means that can be mutually separated in order to interrupt the flow of current in said phase;-- a kinematic mechanism that is operatively connected to said actuation lever and is suitable to move said moving contact means;--electric are quenching means;--a device for protection against short-circuits, which comprises a coil and a moving striker suitable to interact operatively with said kinematic mechanism;-- a device for protection against overloads, which comprises a heat-sensitive element that is suitable to interact operatively with the kinematic mechanism; characterized in that said fixed and moving contact means comprise a first pair of contacts, having a first fixed contact and a first moving contact, and a second pair of contacts, having a second fixed contact and a second moving contact, a first arc quenching chamber and a second arc quenching chamber being respectively associated with said first and second pairs of contacts, said first and second pairs of contacts being electrically series-connected to each other and being crossed by currents that have the same direction.