Composite materials can significantly reduce the weight of current transportation machinery such as cars, trucks and airplanes, and therefore can significantly reduce the energy consumption for driving, and are considered to be important materials for achieving carbon neutrality. Our innovative laser direct joining method is effective for joining dissimilar materials, such as difficult-to-join composite materials and light metals, but the detailed joining mechanism and characteristics have not been clarified until now. Therefore, we conducted research to clarify the detailed joining mechanism of laser direct joining of difficult-to-join composite materials and light metals such as titanium alloys, aluminum alloys and magnesium alloys by combining cutting-edge observation and analysis techniques with materials informatics (MI) and computational science, and to control the joining control factors by optimizing surface modification, etc., in order to achieve ultimate joining strength, excellent reliability, and environmental durability that can be used in space and deep sea. To confirm the joint strength, tensile and peel tests were conducted using test specimens in accordance with ISO 19095. To confirm the reliability, thermal cycle tests of more than 1000 times were conducted. To confirm the environmental durability, deep-sea exposure tests at a depth of more than 1000 m were conducted for more than 6 months, and space exposure tests on the ISS (International Space Station) were conducted for more than 6 months. We will partly report the results that have been obtained so far.