A machine learning system (500) includes a client device (200) that has its own unique learning data and a server device (100) that has training data. Each of the client device (200) and the server device (100) has an AI model. Further, the machine learning system (500) proceeds with learning while keeping the learning data of the client device (200) and the training data of the server device (100) private to each other. The server device (100) includes a fraudulent client detection unit (2530) to estimate a fraudulent client device that performs an attack to disrupts the learning of the AI model in the server device (100).