SuperKEKB, a 7-GeV electron -- 4-GeV positron double-ring collider, was constructed by upgrading KEKB to significantly push the luminosity frontier toward more detailed experiments seeking new discoveries in physics beyond the Standard Model. The design luminosity of SuperKEKB is 8x10^{35} /cm^{2}s, 40 times higher than that achieved in KEKB. The major part of the gain results from significantly decreasing the beam sizes at the interaction point based on the nanobeam collision scheme; the design beam currents in both rings are double those achieved in KEKB. Large-scale construction for the upgrade started in 2010. After the Phase 1 beam commissioning for the upgraded rings without beam collision conducted in 2016, renovation of the interaction region, including the installation of the final-focus superconducting magnets and the Belle II detector was conducted. After completing the construction of SuperKEKB, beam tuning for collision and physics data taking by the Belle II detector started in 2018. SuperKEKB started physics run operation, pioneering luminosity frontier. In this paper, the design, construction, and beam commissioning of SuperKEKB are reviewed, mainly focusing on the accelerator.