Autism spectrum disorder (ASD) is a range of neurodevelopmental conditions generally characterized by impaired social communication and intense interests, and these symptoms are observed from early childhood in ASD. Although ASD has a strong genetic basis and numerous candidate genes have been located, only small effects seem to be attributed to any particular gene. Therefore, the involvement of environmental (non-genetic) factors in the development or exacerbation of ASD cannot be overlooked. Recently, the gut microbiota has attracted attention as an environmental (non-genetic) factor that can cause or exacerbate ASD, and many analyses of the gut microbiota of ASD children have been performed. However, the only consistent finding from these studies is that the gut microbiota of ASD children is clearly different from that of typically developing children, and the details of the relationship between gut microbiota and ASD are not yet clear. In this presentation, we introduce our studies investigating ASD-like behaviors, the gut microbiota, gene expression in the brain, and their relationship in mice treated with antibiotics in early life. We would like to discuss the impact of antibiotic use in early life as a risk factor that disrupts the gut microbiota, and how it affects the neurodevelopment.