Deficiency of pyridoxal 5'-phosphate (PLP) causes neurological abnormalities. The decline in blood PLP concentration has been associated with the onset of dementia, but no studies have been conducted on Japanese. This study aimed to determine the relationship between serum PLP concentration and Alzheimer's disease (AD) in older Japanese individuals. A total of 266 participants, 84 healthy participants, 87 participants with mild cognitive impairment (MCI), and 95 patients with AD were randomly selected from those who visited the National Center for Geriatrics and Gerontology for Longevity Sciences. Serum PLP concentration was significantly lower in the AD compared to the NC. Participants were divided into quartiles (Q1-Q4). The relationship between quartiles of serum PLP concentration and cognitive function was analyzed using logistic regression analyses adjusted with covariate factors (sex, age, number of applicable frailty evaluations, , educational level, albumin, homocysteine, vitamin B, B, and folate). The odds ratios (ORs) for MCI or AD were significantly lower for Q3 and Q4 compared to Q1, and the ORs (95% CI) for Q3 and Q4 were 0.40 (0.16-0.98) and 0.37 (0.13-0.99), respectively. Lower serum PLP concentration is independently related to the incidence of MCI or AD. Further research is needed to clarify the causal relationship.