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The FeSe_1-xS_x superconductors involving non-magnetic nematic phase and its quantum criticality provide a unique platform to investigate the relationship between nematicity and superconductivity. The lifting of superconducting gap nodes due to twin boundaries has been observed experimentally, which suggests indirect evidence for time reversal symmetry breaking (TRSB). It is consistent with the theoretical prediction that the superconducting order parameter breaks the time-reversal symmetry near the nematic twin boundaries. We have performed the muon spin rotation (μSR) measurements on FeSe and observed the spontaneous internal field below the superconducting transition temperature T_c, providing evidence for TRSB state in FeSe. Here we extend zero-field μSR studies to tetragonal FeSe_1-xS_x (x=0.18, 0.2) without nematicity. We find that the μSR relaxation rate starts to grow below T_c in these crystals. This indicates that weak but finite internal magnetic field is induced in the superconducting state, providing strong evidence for TRSB state not only near twin boundaries but also inside the bulk of FeSe_1-xS_x.