Drosophila, as known as fruit fly, is a holometabolous insect typically showing four life-stages ― embryo, larva, pupa, and adult. A fruit fly is often used as a model animal in gene manipulation research since it is easily bred and manipulated with genetic operation. However, detection of the life-stages is manually performed in laboratories. In this paper, we propose the automatic life-stage detection system. We trained the neural network specializing in a semantic segmentation task. As a result, the network could detect pixel-wise an animal in a picture. Moreover, simple signal processing achieved precise detection of life-stage transition comparable with manual operation. It is expected that our system promotes high-throughput screening in genetic research manipulating drosophila.