A bandwidth reservation apparatus and bandwidth reservation method that enable a communication apparatus to quickly begin and stably carry out a data transmission, and a communication apparatus and communication system including the bandwidth reservation apparatus. The bandwidth reservation apparatus includes: a transmit data buffer unit; a bit rate reporting unit; a first reserved bandwidth quantity calculating unit that calculates a first reserved bandwidth quantity required for the transmission on the communication network; a second reserved bandwidth quantity calculating unit that calculates a second reserved bandwidth quantity that is additionally acquired so as to compensate for a shortage of transmission bandwidth that occurs during an interval from a time when the bit rate of the transmit data changes until the first reserved bandwidth quantity can be reserved; and a bandwidth reservation control unit that reserves a combined reserved bandwidth quantity obtained by combining the first and second reserved bandwidth quantities.