As the friction of rolling bearing is composed of the frictions at contact surfaces between rolling elements, raceways and retainer, each one of them should be investigated under respective operating conditions. In this paper, the mechanism of bearing friction of tapered roller bearing at high speed is analyzed theoretically by comparing with experiments. As a result, the fundamental factors composing bearing friction are divided into two parts ; (1) The friction to which the rolling friction at the contact surfaces between rollers and raceways is attributed. (2) The friction to which the sliding friction between rollers and retainer or shoulder of inner race is attributed. At low speed including starting, the latter factor is the main part of the friction. But, at high speed, the bearing friction mostly consists of former factor and it is almost fluid friction of oil film between rollers and raceways, so that the friction of tapered roller bearing at high speed may be approximately expressed with hydrodynamic theory of friction.