In quantum physics, measurement plays a central role. For instance, it is well known that projective (von Neumann) measurement on a quantum state causes irreversible collapse (or reduction) of the wave function. In recent years, it has been known that there exists ‘weak’ measurement where the state collapse is not irreversible. In this talk, I will introduce (successfully, I hope) the concept of weak measurement and explain how weak measurement can be used for protecting quantum states from decoherence.