A method of producing a boron nitride polycrystal, the method comprising: a first step of obtaining a thermally treated powder comprising one or more of hexagonal boron nitride, amorphous boron nitride, rhombohedral boron nitride and boron nitride having a turbostratic structure by performing a thermal treatment onto a powder of a high pressure phase boron nitride comprising one or more of cubic boron nitride, wurtzite type boron nitride and compressed boron nitride at more than or equal to 1300°C; and a second step of obtaining a boron nitride polycrystal by sintering the thermally treated powder under a condition of 8 to 20 GPa and 1200 to 2300°C, wherein the boron nitride polycrystal comprising cubic boron nitride, from 0.01 to 0.5 volume % of compressed hexagonal boron nitride being a hexagonal boron nitride with a narrower plane interval in the C-axis than an ordinary hexagonal boron nitride and optionally: up to 70 volume % wurzite type boron nitride; where the volume ratio of each composition contained in the polycrystalline body of boron nitride is confirmed by the X-ray diffraction method, as explained in the description, less than 0.1 volume % in total of normal pressure type boron nitride, normal pressure type boron nitride being a hexagonal boron nitride, a rhombohedral boron nitride, a boron nitride having a turbostratic structure, and an amorphous boron nitride; and impurities in a total amount of less than 2000 ppma and less than 1000 ppma of each impurity, ppma being parts per million atomic, where the content of impurities can be measured by secondary ion mass spectrometry (SIMS), as described in the description, the cubic boron nitride having an average grain size of less than or equal to 25 nm, found by an intercept method employing a transmission electron microscope, as described in the description, a ratio b/a being less than or equal to 0.07 in measurement of Knoop hardness at 23°C±5°C under a test load of 4.9 N, the ratio b/a being a ratio between a length a of a longer diagonal line and a length b of a shorter diagonal line of a Knoop indentation. a first step of obtaining a thermally treated powder comprising one or more of hexagonal boron nitride, amorphous boron nitride, rhombohedral boron nitride and boron nitride having a turbostratic structure by performing a thermal treatment onto a powder of a high pressure phase boron nitride comprising one or more of cubic boron nitride, wurtzite type boron nitride and compressed boron nitride at more than or equal to 1300°C; and a second step of obtaining a boron nitride polycrystal by sintering the thermally treated powder under a condition of 8 to 20 GPa and 1200 to 2300°C, wherein the boron nitride polycrystal comprising cubic boron nitride, from 0.01 to 0.5 volume % of compressed hexagonal boron nitride being a hexagonal boron nitride with a narrower plane interval in the C-axis than an ordinary hexagonal boron nitride and optionally: up to 70 volume % wurzite type boron nitride; where the volume ratio of each composition contained in the polycrystalline body of boron nitride is confirmed by the X-ray diffraction method, as explained in the description, less than 0.1 volume % in total of normal pressure type boron nitride, normal pressure type boron nitride being a hexagonal boron nitride, a rhombohedral boron nitride, a boron nitride having a turbostratic structure, and an amorphous boron nitride; and impurities in a total amount of less than 2000 ppma and less than 1000 ppma of each impurity, ppma being parts per million atomic, where the content of impurities can be measured by secondary ion mass spectrometry (SIMS), as described in the description, the cubic boron nitride having an average grain size of less than or equal to 25 nm, found by an intercept method employing a transmission electron microscope, as described in the description, a ratio b/a being less than or equal to 0.07 in measurement of Knoop hardness at 23°C±5°C under a test load of 4.9 N, the ratio b/a being a ratio between a length a of a longer diagonal line and a length b of a shorter diagonal line of a Knoop indentation. up to 70 volume % wurzite type boron nitride; where the volume ratio of each composition contained in the polycrystalline body of boron nitride is confirmed by the X-ray diffraction method, as explained in the description, less than 0.1 volume % in total of normal pressure type boron nitride, normal pressure type boron nitride being a hexagonal boron nitride, a rhombohedral boron nitride, a boron nitride having a turbostratic structure, and an amorphous boron nitride; and impurities in a total amount of less than 2000 ppma and less than 1000 ppma of each impurity, ppma being parts per million atomic, where the content of impurities can be measured by secondary ion mass spectrometry (SIMS), as described in the description, the cubic boron nitride having an average grain size of less than or equal to 25 nm, found by an intercept method employing a transmission electron microscope, as described in the description, a ratio b/a being less than or equal to 0.07 in measurement of Knoop hardness at 23°C±5°C under a test load of 4.9 N, the ratio b/a being a ratio between a length a of a longer diagonal line and a length b of a shorter diagonal line of a Knoop indentation.