|| Alzheimer’s disease (AD) is a progressive, neurodegenerative disease characterized by memory and language disorder. The accumulation of senile plaques called β-amyloid and neurofibrillary tangles involving protein tau in the brains of AD patients have been considered as two hallmarks of AD. In AD, it is reported that accumulation of β-amyloid may be observed 25 years before onset, supporting early diagnosis and treatment by brain image analysis, because several techniques have recently been developed to detect β-amyloid and tau protein in brains of persons diagnosed with AD. AD patients are usually suffering from other diseases such as diabetes or periodontal disease, and there is accumulating data to show that these diseases associate with the human microbiome, such as gut and oral microbiota. In this report, the relation ship between AD and the human microbiome is reviewed.