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We examined the muon spin relaxation (μSR) of mixed compounds NiC2O4 2⋅(2-methylimidazole)x(H2O)1-x with x=1.0 and 0.49. Although the macroscopic magnetic properties are obviously different from each other, both systems exhibit similar behavior in the muon spin relaxation. In addition, in the x=0.49 (SG) sample, a critical slowing down of spin dynamics was not observed in this μSR measurement, though the spin-glass like freezing was observed in the susceptibility measurements. Qualitative explanation of these anomalous observations is given.