Abstract In 1980, detecting the evidence of the so-called Grand Unified Theory (GUT) became a major challenge for experimentalists. The GUT predicted that protons would decay, and M. Koshiba proposed the Kamioka Nucleon Decay Experiment (Kamiokande), which was originally conceived and designed for the detection of proton decay signals. Kamiokande was a 3000-t water Cherenkov detector located 1000 m underground in the Kamioka mine. The unique feature of this experiment was the use of 1000 photomultiplier tubes with the world's largest diameter of 20 in (∼51 cm). This article describes the story of the Kamiokande experiment from the preparation stage to the observation of the first candidate events for proton decays.