This book is a concise primer of quantum technology aiming at providing a comprehensive material of fundamentals to help beginners understand the common concepts and background theories to technologies for individual quantum systems. Further, it also describes how the concepts and theories are applied to technologies in various systems. This book consists of three parts. The first part looks back over basics of quantum mechanics necessary for the main content, including quantum state and operators, time evolution and perturbation theory. The second part explains in detail key components indispensable to follow quantum technologies: two-level systems, harmonic oscillator and cavity quantum electrodynamics and resonators. In the third part, the physical quantum systems are treated in a more abstract way by introducing quantum logic gates, quantum measurement and quantum error correction. Technical supplements are included in Appendices. The well-compiled topics and concise presentation feature the book as a supplemental primer in the courses of quantum technologies including quantum computing, quantum communication, quantum sensing and quantum simulation.