Provided are a material testing machine and a gripping force detecting method that can easily judge whether a test piece is gripped with an appropriate gripping force by a gripper. A controlling section is connected to a FFT transforming section via a load cell; the FFT transforming section calculates a natural frequency of a system comprising a test piece and an upper gripper which is connected to a load cell based on a detected value of a force of the load cell. In addition, the controlling section is connected to a storing section which stores the natural frequency calculated by the FFT transforming section. Furthermore, the controlling section is also connected to a comparing section which compares the natural frequency calculated by the FFT transforming section and the natural frequency stored by the storing section before a test starts.