Abstract: A cell disassembly unit, cell assembly unit, and clock reproduction method which enables a clock of a source signal multiplexed and transmitted to be stably reproduced even in a situation where a synchronization clock of a transmission network is unstable. The source signal multiplexed into cells is inputted in accordance with the clock which is asynchronous to a network. A timing information separation circuit included in a cell disassembly unit separates transmission timing information which is obtained by converting transmission timing in each constant period of the clock of the source signal by the use of transmission timing of a synchronization clock of the network from the cells received. A timing information replacement circuit accepts the transmission timing information separated, averages differentials between adjacent pieces of transmission timing information every predetermined period, and replaces the transmission timing information on the basis of an average differential.