A hexagonal boron nitride powder comprising an aggregate of primary particles of hexagonal boron nitride, wherein the powder has a primary particle size, determined by SEM, of less than 10 μm, a ratio of an average longer diameter (L 1), determined by SEM, to an average thickness (d 1) of the primary particles, determined by SEM, [L 1 /d 1], of 5.0 or more and 20 or less, a BET specific surface area, determined as described in "BET Specific Surface Area of BN Fine Powder and of hBN Powder)" in the description, of 3.0 m 2 /g or more and 5.0 m 2 /g or less and a bulk density, determined as described in "(Bulk Density of hBN Powder)" in the description, of 0.75g/cm 3 or more, wherein the "average longer diameter" means a number average value of the longer diameters of the primary particles, the longer diameter meaning the maximum diameter in a planar direction of the primary particle, and the "average thickness" means a number average value of the thicknesses of the primary particles, and the powder has one maximum peak in a range of a particle size of 45 μm or more and 150 μm or less in a particle size distribution curve, measured using a particle size distribution analyzer by the laser diffraction scattering method, of the hexagonal boron nitride powder which has been classified using a sieve having an opening of 106 μm and a sieve having an opening of 45 μm with the two sieves piled up to have a particle size of 45 μm or more and 106 μm or less, and has a decrease rate of the maximum peak of 10% or more and less than 40% when a dispersion liquid obtained by dispersing 0.06g of the hexagonal boron nitride powder in 50g of water is placed in a 50 ml container and subjected to an ultrasonic treatment for 1 minute under conditions of an output of 150 W and an oscillating frequency of 19.5 kHz, and thereafter the measurement of the particle size distribution was conducted while stirring the dispersion liquid using a magnetic stirrer under a condition of a number of revolutions of 400 rpm, the peak decrease rate being calculated by the following expression: Peak decrease rate % = maximum peak height before treatment a − maximum peak height after treatment b / maximum peak height before treatment a × 100 the powder has a primary particle size, determined by SEM, of less than 10 μm, a ratio of an average longer diameter (L 1), determined by SEM, to an average thickness (d 1) of the primary particles, determined by SEM, [L 1 /d 1], of 5.0 or more and 20 or less, a BET specific surface area, determined as described in "BET Specific Surface Area of BN Fine Powder and of hBN Powder)" in the description, of 3.0 m 2 /g or more and 5.0 m 2 /g or less and a bulk density, determined as described in "(Bulk Density of hBN Powder)" in the description, of 0.75g/cm 3 or more, wherein the "average longer diameter" means a number average value of the longer diameters of the primary particles, the longer diameter meaning the maximum diameter in a planar direction of the primary particle, and the "average thickness" means a number average value of the thicknesses of the primary particles, and the powder has one maximum peak in a range of a particle size of 45 μm or more and 150 μm or less in a particle size distribution curve, measured using a particle size distribution analyzer by the laser diffraction scattering method, of the hexagonal boron nitride powder which has been classified using a sieve having an opening of 106 μm and a sieve having an opening of 45 μm with the two sieves piled up to have a particle size of 45 μm or more and 106 μm or less, and has a decrease rate of the maximum peak of 10% or more and less than 40% when a dispersion liquid obtained by dispersing 0.06g of the hexagonal boron nitride powder in 50g of water is placed in a 50 ml container and subjected to an ultrasonic treatment for 1 minute under conditions of an output of 150 W and an oscillating frequency of 19.5 kHz, and thereafter the measurement of the particle size distribution was conducted while stirring the dispersion liquid using a magnetic stirrer under a condition of a number of revolutions of 400 rpm, the peak decrease rate being calculated by the following expression: Peak decrease rate % = maximum peak height before treatment a − maximum peak height after treatment b / maximum peak height before treatment a × 100