Magnetars are regarded as the most magnetized compact objects in the Universe. They are popular sources with many unsolved questions in themselves, but also linked to various high energy phenomena, such as fast radio bursts and super-luminous supernovae. So far, our observational knowledge of this special group has been mainly accumulated from 30 magnetars. Ten among them are found in supernova remnants, demonstrating that they are young pulsars. Since a magnetar and its supernova remnant share a common progenitor and are born in the same explosion, studying them together will result in a better mutual understanding. In this talk, I will present the studies of supernova remnants hosting magnetars and show how these studies unveil the initial condition and formation mechanism of magnetars.
