The 1:1 balun constructed of a bifilar delay line and an inverse 1:1 transformer is analyzed for its input characteristics. As a main result of the analyses, the following results were obtained about the balun in this paper. 1) Resonance occurs when the line length is equal to n (any positive integer) times half of a wavelength of the unbalanced transmission mode. 2) The lower cutoff frequency is determined by electromagnetic coupling coefficient "km" when Lsigma and ι are constant. 3) The upper cutoff frequency is also given by a combination of km and ke, etc. 4) Shortening of the distance between the balun and the ground makes the resonant frequency lower when the balun has a magnetic core or when the electrostatic coupling coefficient "ke" becomes smaller and km is kept constant. ωOI becomes lower. 5) In order to realize a wide-band balun, βb / βu shouId be larger and and βb / ω should be kept constant.