Abstract: Provided is an ion generating device for organic matter decomposition for generating ions to decompose organic matter stored in a tank. The ion generating device includes a needle electrode and a plate electrode, both facing each other, and a direct-current power supply unit configured to apply a direct-current voltage with positive polarity to the needle electrode. The direct-current power supply unit includes a voltage controller configured to set the direct-current voltage to a specified voltage value to produce positive corona discharge between the needle electrode and the plate electrode under atmospheric pressure.