Construction of 275kV and 500kV substation in densely populated area is thought to be necessary in the future to keep stable supply of electricity. Using gas-insulated transformer is now considered for substations because gas-insulated transformers are non-frammable and non-explosive. Since completion of 77kV gas-insulated transformer in 1979, we have continued to research to develope gas-insulated transformers applicable to higher voltage level up to 500kV. As the results of the research, 275kV, 300 MVA liquid cooling type gas-insulated transformer has been developed. We introduce the abstruct of our study.