The energy problem, combined with a shortage of natural energy resources and increasing atmospheric concentrations of CO2, needs to be resolved in the latter half of this century for human sustainability. Considering the climate inertia and the leading time for new energy infrastructure, we urgently have to establish the technologies for clean and safe energy system. As is widely known, there is unlimited constant solar energy supply in space free from the whether conditions, quite different from that on the earth. The concept of the Solar Power Satellite (SPS) is to tap the solar energy using a large-scale photovoltaic array in space and to transmit it to the ground using microwave or laser beam. This energy system is essentially clean and safe. This system will contribute to solving both the energy problems and the global environmental issues. Solar Power Satellite will be the forerunner to the human expansion into space. Currently verifications for SPS technologies are underway on ground. If we start SPS demonstration experiment in space in the early 2020's, we will be able to realize the commercial SPS in the early 2040's.