A miniature circuit breaker comprising: a pair of contact mechanisms that are provided to correspond to a pair of circuits corresponding to a pair of poles and switch the pair of circuits; a switching mechanism that is commonly provided in the pair of contact mechanisms and drives the pair of contact mechanisms to a circuit opening position or a circuit closing position; a trip bar that is rotatable to a first position for latching the switching mechanism in the circuit closing position or to a second position for releasing the switching mechanism to operate to the circuit opening position; and an instant trip mechanism that presses the trip bar to rotate to the second position in response to a fault current on the circuit requiring an instant trip, wherein the instant trip mechanism comprises a pair of armature assemblies that are provided to correspond to the pair of poles and movable to a position for pressing the trip bar to rotate to the second position; a pair of electromagnets that are provided to face the pair of armature assemblies and applies a magnetic attractive force to the pair of armature assemblies in response to the fault current on the circuit requiring an instant trip, characterised in that the trip bar is Y shaped and has an upper branch portion divided into two fork portions and a lower end supported by a support shaft; and each of the pair of armature assemblies comprises a first armature portion that is pivotally supported to be rotatable and has a cam surface portion for pressing the trip bar ; and a second armature portion that is coupled to the first armature so as to be rotatable together and disposed to face the corresponding electromagnet ; and a coupling portion that couples the first armature portion and the second armature portion . a pair of contact mechanisms that are provided to correspond to a pair of circuits corresponding to a pair of poles and switch the pair of circuits; a switching mechanism that is commonly provided in the pair of contact mechanisms and drives the pair of contact mechanisms to a circuit opening position or a circuit closing position; a trip bar that is rotatable to a first position for latching the switching mechanism in the circuit closing position or to a second position for releasing the switching mechanism to operate to the circuit opening position; and an instant trip mechanism that presses the trip bar to rotate to the second position in response to a fault current on the circuit requiring an instant trip, wherein the instant trip mechanism comprises a pair of armature assemblies that are provided to correspond to the pair of poles and movable to a position for pressing the trip bar to rotate to the second position; a pair of electromagnets that are provided to face the pair of armature assemblies and applies a magnetic attractive force to the pair of armature assemblies in response to the fault current on the circuit requiring an instant trip, characterised in that the trip bar is Y shaped and has an upper branch portion divided into two fork portions and a lower end supported by a support shaft; and each of the pair of armature assemblies comprises a first armature portion that is pivotally supported to be rotatable and has a cam surface portion for pressing the trip bar ; and a second armature portion that is coupled to the first armature so as to be rotatable together and disposed to face the corresponding electromagnet ; and a coupling portion that couples the first armature portion and the second armature portion . <EOS>