The anomalous magnetic moment of nucleon is investigated by taking the possible ex­ citations of the nucleon core or the cloud pions into account. For the sake of definiteness and simplicity two assumptions are made: 1) the physical nucleon is composed of the static core accompanied by one or no cloud particle, 2) the virtual excitations can be treated as the-occurrence of the particles having definite spin J, isospin I, parity P and mass with some probability. As the virtual excitations, the occurrences of cores of I =J = 3/2, I= 1/2 .}=3/2,I=1/2}=5/2, etc., and of the cloud particles with 1=}=1, I=1 }=0, etc., are considered. It is shown that the most favourable model for the physical nucleon is the one in which the nucleon is assumed to consist of the cores of I =J = 1/2, P= ( +) and I =J = 3/2 P= (+) with nearly equal weight, the former being accompanied by one or no pion and the latter being accompanied by one pion. By using this model p-wave rc+.p scattering is also discussed for the consistency of the present approach. § 1. Introduction and summary Many attempts11 have -been made to explain the observed values of the magnetic moment of nucleon by making use of various models and approximation methods. Some of these attempts could reproduce at least qualitatively the observed values of the magnetic moment, but the quantitative agreement between .theory and experiment has not yet been achieved _by any of them. The defects of these attempts seem to be due mainly to some over-simplification of the model of nucleon structure as well as to the lack of suitable approximation methods. In recent years, various excited states of nucleon and pions have been discovered, e.g. the so-called (3/2, 3/2) resonance in rr-N scattering, (' and (tJ mesons decaying into several pions, and so on. Although these excited states have. been detected mainly in scattering or production processes, it may be reasonable to suppose that similar virtual excitations occur also in the core and the proper field of nucleon. It seems to us necessary to reExamine various attempts at the problems of the anomalous magnetic moment of nucleon by con­ sidering the effects of these virtual excitations. In order to explain the observed values of the magnetic moment Sachs2l proposed a model of the physical nucleon in which the nucleon is assumed to consist of a " nucleore " of spin 1/2 surrounded by the pion clouds. He showed that the observed values of the magnetic moment cannot be explained if no more than one pion occurs with an appreciable probability, or to be more specific the two-pion state with suitable spatial distribution is requi.red.