A miniature circuit breaker comprising: a pair of contact mechanisms (410, 510, 610, 710) 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 (420, 620, 720) that is commonly provided in the pair of contact mechanisms (410, 510, 610, 710) and drives the pair of contact mechanisms (410, 510, 610, 710) to a circuit opening position or a circuit closing position; a trip bar (490, 590, 690, 790) that is rotatable to a first position for latching the switching mechanism (420, 620, 720) in the circuit closing position or to a second position for releasing the switching mechanism (420, 620, 720) to operate to the circuit opening position; and an instant trip mechanism (430, 530, 630, 730) that presses the trip bar (490, 590, 690, 790) to rotate to the second position in response to a fault current on the circuit requiring an instant trip, wherein the instant trip mechanism (430, 530, 630, 730) comprises a pair of armature assemblies (460, 560, 660, 760) that are provided to correspond to the pair of poles and movable to a position for pressing the trip bar (490, 590, 690, 790) to rotate to the second position; a pair of electromagnets (450, 550, 650, 750) that are provided to face the pair of armature assemblies (460, 560, 660, 760) and applies a magnetic attractive force to the pair of armature assemblies (460, 560, 660, 760) in response to the fault current on the circuit requiring an instant trip, characterised in that the trip bar (490, 590, 690, 790) 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 (460, 560, 660, 760) comprises a first armature portion (480, 580, 680, 780) that is pivotally supported to be rotatable and has a cam surface portion (487, 587, 687, 787) for pressing the trip bar (490, 590, 690, 790); and a second armature portion (470, 570, 670, 770) that is coupled to the first armature so as to be rotatable together and disposed to face the corresponding electromagnet (450, 550, 650, 750); and a coupling portion (481, 581, 681, 781) that couples the first armature portion (480, 580, 680, 780) and the second armature portion (470, 570, 670, 770). a pair of contact mechanisms (410, 510, 610, 710) 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 (420, 620, 720) that is commonly provided in the pair of contact mechanisms (410, 510, 610, 710) and drives the pair of contact mechanisms (410, 510, 610, 710) to a circuit opening position or a circuit closing position; a trip bar (490, 590, 690, 790) that is rotatable to a first position for latching the switching mechanism (420, 620, 720) in the circuit closing position or to a second position for releasing the switching mechanism (420, 620, 720) to operate to the circuit opening position; and an instant trip mechanism (430, 530, 630, 730) that presses the trip bar (490, 590, 690, 790) to rotate to the second position in response to a fault current on the circuit requiring an instant trip, wherein the instant trip mechanism (430, 530, 630, 730) comprises a pair of armature assemblies (460, 560, 660, 760) that are provided to correspond to the pair of poles and movable to a position for pressing the trip bar (490, 590, 690, 790) to rotate to the second position; a pair of electromagnets (450, 550, 650, 750) that are provided to face the pair of armature assemblies (460, 560, 660, 760) and applies a magnetic attractive force to the pair of armature assemblies (460, 560, 660, 760) in response to the fault current on the circuit requiring an instant trip, characterised in that the trip bar (490, 590, 690, 790) 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 (460, 560, 660, 760) comprises a first armature portion (480, 580, 680, 780) that is pivotally supported to be rotatable and has a cam surface portion (487, 587, 687, 787) for pressing the trip bar (490, 590, 690, 790); and a second armature portion (470, 570, 670, 770) that is coupled to the first armature so as to be rotatable together and disposed to face the corresponding electromagnet (450, 550, 650, 750); and a coupling portion (481, 581, 681, 781) that couples the first armature portion (480, 580, 680, 780) and the second armature portion (470, 570, 670, 770).