The subject of this paper is concerned mainly with the formation and development of Faraday's concept of the lines of magnetic force from 1845 to 1855, during which he concentrated his efforts mostly upon the researches on magnetic phenomena. First, this paper shows that the concept of lines of magnetic force, reintroduced in 1845, was given double meanings: one was as a conceptual tool representing the direction of the magnetic action; the other was as a physical reality transmitting the magnetic force. Then, after the limitations of each of these meanings were removed, this concept emerged as a physical reality essential to the magnetic phenomena. The paper also shows that this removal was made possible by his experimental researches at 1850, concerning the relationship between the magnetic force and vacuum. Chapter 1, 2, 3 of the main part of the paper treat the above topics in detail. In Chapter 4, Faraday's experiments to establish the physical existence of the lines of magnetic force are discussed, and the problems of these experiments caused by his belief in the physical existence of it are considered.