This textbook for beginners summarizes the basic content of linear algebra that students specializing in the field of natural science should know. The book is designed to meet the needs of those who use linear algebra in the field, namely, to first learn mathematics ‘‘as a tool,’’ ‘‘as a language for describing phenomena,’’ and ‘‘for immediate use,’’ omitting proofs and the like as much as possible and not being too concerned with logical rigor, while keeping the main points of linear algebra easy to understand. The book is explained in the following manner. In the third edition, a new chapter has been added to explain what vectors are. (Translated with www.DeepL.com/Translator (free version))