We developed a timing synchronization system for injection from the linac of the XFEL machine, SACLA, to the storage ring (SR) of SPring-8. This injection scheme is demanded by the low-emittance upgraded ring in the future. Since the RF frequencies of the linac and the SR do not have an integer multiples relation, we have to introduce a new scheme to synchronize the beam ejection timing of the linac to the desired RF bucket timing of the SR. The new system was implemented to the MTCA.4 standard modules. The control of the bucket address is done according to the pre-defined injection table. The system was installed to the linac and the measured timing jitter was better than 4 ps in rms, which was enough to obtain high injection efficiency. This injection scheme has been adopted for the user operation of SPring-8 since February 2020.