A gas insulated switchgear comprising: a basic unit portion configured by a standard unit and a variable unit portion, the basic unit portion including: - a first pressure tank in which insulating gas is sealed, and a first opening portion (1c1) is provided in at least one surface of the first pressure tank; - a circuit breaker which is arranged in the inside of the first pressure tank, and configured to interrupt a large current; - a first disconnecting switch which is connected to the circuit breaker, and configured to disconnect a main circuit; and - a first bushing configured to connect the main circuit to an external apparatus, and the variable unit portion including: - a busbar which is connected to one side terminal of the circuit breaker; - a second pressure tank in which insulating gas is sealed, a second opening portion (1d1) facing the first opening portion (1c1) is provided in at least one surface of the second pressure tank, wherein the busbar is provided in the second pressure tank; - one or more second bushings passing through a wall of the second pressure tank, and configured to connect an internal main circuit of the second pressure tank to an external apparatus; and - one or a plurality of second disconnecting switches which is arranged between the busbar and the one or more second bushings, and disconnects the main circuit, characterised in that: the first opening portion (1c1) of the first pressure tank and the second opening portion (1d1) of the second pressure tank are butted and joined facing each other to form into one constitutional body so that the inside of the first pressure tank and the second pressure tank is configured as one gas compartment, wherein the first opening portion (1c1) is made small by making the bottom surface (1c2) of the first pressure tank of the basic unit portion incline and the second opening portion (1d1) of the second pressure tank of the variable unit portion is made small by adjusting to the first opening portion (1c1). the basic unit portion including: - a first pressure tank in which insulating gas is sealed, and a first opening portion (1c1) is provided in at least one surface of the first pressure tank; - a circuit breaker which is arranged in the inside of the first pressure tank, and configured to interrupt a large current; - a first disconnecting switch which is connected to the circuit breaker, and configured to disconnect a main circuit; and - a first bushing configured to connect the main circuit to an external apparatus, and - a first pressure tank in which insulating gas is sealed, and a first opening portion (1c1) is provided in at least one surface of the first pressure tank; - a circuit breaker which is arranged in the inside of the first pressure tank, and configured to interrupt a large current; - a first disconnecting switch which is connected to the circuit breaker, and configured to disconnect a main circuit; and - a first bushing configured to connect the main circuit to an external apparatus, and the variable unit portion including: - a busbar which is connected to one side terminal of the circuit breaker; - a second pressure tank in which insulating gas is sealed, a second opening portion (1d1) facing the first opening portion (1c1) is provided in at least one surface of the second pressure tank, wherein the busbar is provided in the second pressure tank; - one or more second bushings passing through a wall of the second pressure tank, and configured to connect an internal main circuit of the second pressure tank to an external apparatus; and - one or a plurality of second disconnecting switches which is arranged between the busbar and the one or more second bushings, and disconnects the main circuit, - a busbar which is connected to one side terminal of the circuit breaker; - a second pressure tank in which insulating gas is sealed, a second opening portion (1d1) facing the first opening portion (1c1) is provided in at least one surface of the second pressure tank, wherein the busbar is provided in the second pressure tank; - one or more second bushings passing through a wall of the second pressure tank, and configured to connect an internal main circuit of the second pressure tank to an external apparatus; and - one or a plurality of second disconnecting switches which is arranged between the busbar and the one or more second bushings, and disconnects the main circuit, characterised in that: the first opening portion (1c1) of the first pressure tank and the second opening portion (1d1) of the second pressure tank are butted and joined facing each other to form into one constitutional body so that the inside of the first pressure tank and the second pressure tank is configured as one gas compartment, wherein the first opening portion (1c1) is made small by making the bottom surface (1c2) of the first pressure tank of the basic unit portion incline and the second opening portion (1d1) of the second pressure tank of the variable unit portion is made small by adjusting to the first opening portion (1c1). the first opening portion (1c1) of the first pressure tank and the second opening portion (1d1) of the second pressure tank are butted and joined facing each other to form into one constitutional body so that the inside of the first pressure tank and the second pressure tank is configured as one gas compartment, wherein the first opening portion (1c1) is made small by making the bottom surface (1c2) of the first pressure tank of the basic unit portion incline and the second opening portion (1d1) of the second pressure tank of the variable unit portion is made small by adjusting to the first opening portion (1c1).