William Gilbert's De magnete, which was published in 1600, has a very peculiar place in the history of science. This article aims at understanding the structure and character of Gilbert's magnetical philosophy on the basis of detailed analysis of De magnete. For the first step, Gilbert's experimental studies are examined and the modernity of his experimental method is revealed. Next, his matter theory, which was coined to explain the cause of magnetic phenomena and electrostatic phenomena, and his cosmological arguements are considered respectively. In conclusion, the discussions mentioned above are put together to trace the internal structure of Gilbert's magnetical philosophy, and it is shown that the hypothesis that the earth is a great loadstone had fundamental position in the structure. The article ends with the remark that Gilbert's magnetical philosophy, which represented a characteristic trend of the late 16th century European science, had more complicated character and route of formation than has been viewed.