In cogeneration system, the pre-chamber natural gas engine adopts combustion technologies such as ultra-high supercharged lean burn and Miller cycle to increase the theoretical efficiency by increasing the specific heat ratio and the mechanical efficiency by improving the specific power. In recent years, the use of hydrogen fuel has been attracting attention in order to achieve carbon neutrality, and it is required to operate existing high-efficiency natural gas engines by appropriately mixing hydrogen. For this purpose, it is important to have natural gas and hydrogen co-combustion technology that allows combustion at any mixture ratio without major modifications. The authors mixed hydrogen into the fuel of an ultra-high supercharged lean burn pre-chamber natural gas engine (Bore size: 200mm) that has already achieved high efficiency and performed combustion experiments at BMEP (Brake mean effective pressure) of 2 MPa or more. The engine load and hydrogen mixture ratio were used as operating parameters to search for the optimal air excess ratio and ignition timing. Abnormal combustion such as pre-ignition and thud occurs in the high load and high hydrogen mixture regions, but the occurrence regions may or may not overlap, and it was found that this limits the operable limit on the high load and high hydrogen mixture sides. We reported on the operating range in which each type of abnormal combustion occurs, the relationship between the frequency of occurrence and operating conditions, the fact that the pattern of abnormal combustion occurrence is not continuous, the relationship and continuity with the cycles before and after, and the fact that pre-ignition is less likely to occur when the pre-chamber excess air ratio is large. Furthermore, we investigated a method of separating the main chamber pressure oscillation caused by the ejection jet from pre-chamber combustion, known as ringing, from the pressure oscillation caused by knocking. We have set a threshold for the maximum amplitude of high-frequency components and separating knocking from ringing by the frequency of occurrence.