To study populations of non-thermal and hot thermal electrons, we propose TOO observations of two black hole binaries (BHBs), XTE J1550-564 and GROJ1655-40, in the very high state (VHS). These two sources are well studied BHBs, and their VHS can be clearly distinguished based on the RXTE/ASM. Though origin of non-thermal hard X-ray emission which is generally observed in high accreting BHBs has long been a great mystery for black hole physics, in the VHS, the location of at least the hot-thermal electrons has been found to exist closely to the optically thick accretion disk like disk corona. By the good spectra at 50-500keV with the HXD, we can distinguish whether the non-thermal electrons have same population as the hot-thermal electrons or not.