To realize a transmission data switching apparatus without short break and an automatic phasing system using the same which readjusts the phase to minimum essentials, so as to hardly bring about short break due to phase readjustment.

The transmission data switching apparatus without short break is composed of a first and second system modules 110, 120 and a selector 14. Modules 11, 12 are composed of delay controlling memories 111, 121, synchronous detector circuits 112, 122, write address counters 113, 123, variable delay circuits 114, 124, read address counters 115, 125 and phase decision controllers 116, 126, respectively. They use phase fixing functions together. When the phase fixing is canceled, a re-phasing occurs and, if the working system is in short interruption, the transmission route length of the standby system is changed to a correct value and re-inserted to avoid short break of the working system.