Alachlor is a chloroacetanilide herbicide used to control annual grasses in grazing system and has been observed along with its metabolites in animal foodstuffs. Its residue violations in animal foodstuffs are increasing significantly and therefore, useful and efficient assay is required. In this study, a rapid, useful, routine, and sensitive liquid chromatography with tandem mass spectrometry for simultaneously determining and confirming alachlor and its metabolites (2-chloro-N-(2,6-diethylphenyl) acetamide, 2,6-diethylaniline, and 2-ethyl-6-(1-hydroxyethyl) aniline) in beef muscle, liver, milk, and egg samples was examined. The proposed method uses a simple extraction with methanol followed by dispersive and solid-phase extraction cleanup. Observably, absolute calibration curves were linear from the limit of quantification to 100 ng/mL (r(2) > 0.999). The recovery values fortified at 0.02 and 0.2 mu g/g ranged from 70.4 to 107.6% with a relative standard deviation <= 7.1%. Alachlor and its metabolites in applied samples from Japanese markets were undetected using developed method. We examined the useful and efficient method and sample preparation scheme for actual and rational process of alachlor's residue.