JT-25D Digital Insulation Tester, also known as a megohmmeter.
It is used for measuring the insulation resistance of various materials and electrical equipment like transformers.
The device has a rated insulation test voltage of 1000V.
It can measure resistance in the range of 0-2000M$\Omega$ (megaohms).
The device features a large LCD display and includes functions like data storage and automatic shutdown