A circuit breaker includes a tank grounded and filled with an insulating gas, a first electrical series-connected section having a main conductor and a main breaking section, and a second electrical series-connected section having a resistor and an advance closing contact device. The first electrical series-connected section is coupled electrically in parallel to the second electrical series-connected section. The fixed contact side of the main breaking section is supported by an insulating cylinder and the fixed contact side of the advance closing contact device is supported by an insulating plate from the fixed contact side of the main breaking section. A breaking unit, chiefly composed of the main breaking section and the advance closing contact device, is supported by insulating cylinders. A resistor unit is supported by an insulating cylinder. The assembly of each of the units can be performed completely outside the tank, thereby eliminating the need for performing any adjustment type of work within the tank.