AbstractIn order to measure velocity distribution functions of end-loss ions in the tandem mirror, a diagnostic device named ELECA (End-Loss Energy Component Analyzer) has been developed. The ELECA enables us to measure simultaneously both the pitch angle and the energy of the end-loss ions without mixing of the high energy electrons even during strong plugging. By sweeping the electrode voltages of the analyzer with high frequency, time evolution of the ion distribution in the loss region is obtained during one shot. The ELECA was located on one end of the tandem mirror GAMMA 10, and it was found that a complicated structure appeared in the distribution functions of the end-loss ions. The structure is caused by the effects of the electrostatic potential, the strong magnetic field and ion transport in the velocity space. It was also verified from a microscopic viewpoint that the confining potential was created by turning on the electron cyclotron resonance heating (ECRH) without the production of sloshing ions by neutral beam injection.