A method of washing lumps of poly crystalline silicon comprising: a step of acid cleaning in which the lumps of poly crystalline silicon retained in a basket is cleaned with an acid solution by immersing the lumps of poly crystalline silicon retained in the basket in the acid solution held in an acid cleaning bath; and a step of water cleaning in which the lumps of poly crystalline silicon retained in the basket is cleaned with pure water after the step of acid cleaning, wherein, residual of the acid solution on a surface of the lumps of polycrystalline silicon is removed by immersing the lumps of polycrystalline silicon retained in the basket in pure water held in a water cleaning bath, and replacing the pure water in the water cleaning bath by fresh pure water at least once, in the step of water cleaning, an electrical conductivity C of the fresh pure water in the water cleaning bath is measured after allowing the lumps of polycrystalline silicon to stand and to remain immersed in the fresh pure water, for at least 2 hours, the specific resistance of pure water which is supplied to the water cleaning bath is 15 MΩ·cm or more, the step of the water cleaning is finished after the electrical conductivity C of the fresh pure water becomes 2 μS/cm or lower, the water cleaning bath comprises: a drain outlet that is provided on an end portion of the water cleaning bath in a longitudinal direction and drains the pure water from the water cleaning bath; a feed-water inlet that is provided in an opposite end portion of the water cleaning bath to the end portion in the longitudinal direction and supplies the fresh pure water to the water cleaning bath; and a plurality of platforms that is fixed at a bottom plate of the water cleaning bath and supports the basket retaining the lumps of polycrystalline silicon, the bottom plate of the water cleaning bath is sloped down from the opposite end portion to the end portion of the water cleaning bath in the longitudinal direction, and each of support surfaces of the plurality of the platforms is sloped in a width direction of the water cleaning bath. a step of acid cleaning in which the lumps of poly crystalline silicon retained in a basket is cleaned with an acid solution by immersing the lumps of poly crystalline silicon retained in the basket in the acid solution held in an acid cleaning bath; and a step of water cleaning in which the lumps of poly crystalline silicon retained in the basket is cleaned with pure water after the step of acid cleaning, wherein, residual of the acid solution on a surface of the lumps of polycrystalline silicon is removed by immersing the lumps of polycrystalline silicon retained in the basket in pure water held in a water cleaning bath, and replacing the pure water in the water cleaning bath by fresh pure water at least once, in the step of water cleaning, an electrical conductivity C of the fresh pure water in the water cleaning bath is measured after allowing the lumps of polycrystalline silicon to stand and to remain immersed in the fresh pure water, for at least 2 hours, the specific resistance of pure water which is supplied to the water cleaning bath is 15 MΩ·cm or more, the step of the water cleaning is finished after the electrical conductivity C of the fresh pure water becomes 2 μS/cm or lower, the water cleaning bath comprises: a drain outlet that is provided on an end portion of the water cleaning bath in a longitudinal direction and drains the pure water from the water cleaning bath; a feed-water inlet that is provided in an opposite end portion of the water cleaning bath to the end portion in the longitudinal direction and supplies the fresh pure water to the water cleaning bath; and a plurality of platforms that is fixed at a bottom plate of the water cleaning bath and supports the basket retaining the lumps of polycrystalline silicon, the bottom plate of the water cleaning bath is sloped down from the opposite end portion to the end portion of the water cleaning bath in the longitudinal direction, and each of support surfaces of the plurality of the platforms is sloped in a width direction of the water cleaning bath.