A gas-insulated switchgear housing a main bus-bar conductor and accommodating a three-position disconnector switch/earthing switch, comprising: a disconnector switch fixed contact portion; an earthing switch fixed contact portion; a movable contactor supporting conductor disposed between both the contact portions; and a movable contactor slidably supported by the movable contactor supporting conductor to be connected or separated to/from both the contact portions, in a gas container to which insulation gas is filled, wherein the gas container houses the main bus-bar conductor and has two facing openings (1a, 1b) in a longitudinal direction of the main bus-bar conductor and at least one opening (1c, 1d) in a direction almost orthogonal to the longitudinal direction of the main bus-bar conductor; wherein a disconnector switch fixed side unit is mounted to one of the two facing openings provided in the gas container, the disconnector switch fixed side unit having a first insulation spacer and the main bus-bar conductor whose one end is fixed to the first insulation spacer, in a center portion of the main bus-bar conductor, the disconnector switch fixed contact portion in a direction orthogonal to the longitudinal direction of the main bus-bar conductor being formed, and in the other end portion of the main bus-bar conductor a connection portion to another conductor being formed; wherein an earthing switch fixed side unit is mounted to a mounting surface side of the disconnector switch fixed side unit of the gas container, the earthing switch fixed side unit having the earthing switch fixed contact portion and an external lead-out terminal to connect the earthing switch fixed contact portion to the outside; and wherein a movable contactor supporting unit is mounted to the opening provided in a direction almost orthogonal to the longitudinal direction of the main bus-bar conductor of the gas container, the movable contactor supporting unit having a second insulation spacer, the movable contactor supporting conductor fixed to the second insulation spacer and disposed to face the disconnector switch fixed contact portion, and the movable contactor slidably supported by the movable contactor supporting conductor to be connected or separated to/from either one of the disconnector switch fixed contact portion and the earthing switch fixed contact portion, characterized in that a detachable unit is mounted to the other facing opening of the gas container, the detachable unit having a third insulation spacer mounted to be able to move forward and backward in an internal-direction of the gas container, and a detachable conductor fixed to the third insulation spacer and connectable or separatable to/from the connection portion formed in the other end portion of the main bus-bar conductor. a disconnector switch fixed contact portion; an earthing switch fixed contact portion; a movable contactor supporting conductor disposed between both the contact portions; and a movable contactor slidably supported by the movable contactor supporting conductor to be connected or separated to/from both the contact portions, in a gas container to which insulation gas is filled, wherein the gas container houses the main bus-bar conductor and has two facing openings (1a, 1b) in a longitudinal direction of the main bus-bar conductor and at least one opening (1c, 1d) in a direction almost orthogonal to the longitudinal direction of the main bus-bar conductor; wherein a disconnector switch fixed side unit is mounted to one of the two facing openings provided in the gas container, the disconnector switch fixed side unit having a first insulation spacer and the main bus-bar conductor whose one end is fixed to the first insulation spacer, in a center portion of the main bus-bar conductor, the disconnector switch fixed contact portion in a direction orthogonal to the longitudinal direction of the main bus-bar conductor being formed, and in the other end portion of the main bus-bar conductor a connection portion to another conductor being formed; wherein an earthing switch fixed side unit is mounted to a mounting surface side of the disconnector switch fixed side unit of the gas container, the earthing switch fixed side unit having the earthing switch fixed contact portion and an external lead-out terminal to connect the earthing switch fixed contact portion to the outside; and wherein a movable contactor supporting unit is mounted to the opening provided in a direction almost orthogonal to the longitudinal direction of the main bus-bar conductor of the gas container, the movable contactor supporting unit having a second insulation spacer, the movable contactor supporting conductor fixed to the second insulation spacer and disposed to face the disconnector switch fixed contact portion, and the movable contactor slidably supported by the movable contactor supporting conductor to be connected or separated to/from either one of the disconnector switch fixed contact portion and the earthing switch fixed contact portion, characterized in that a detachable unit is mounted to the other facing opening of the gas container, the detachable unit having a third insulation spacer mounted to be able to move forward and backward in an internal-direction of the gas container, and a detachable conductor fixed to the third insulation spacer and connectable or separatable to/from the connection portion formed in the other end portion of the main bus-bar conductor.