The invention provides an effective method for subjecting the inner surface of a tubular body made of a plastic resin to a treatment with low temperature plasma according to which the tubular body is dipped in an insulating oil bath while the inside of the tubular body is filled with a plasma gas under a reduced pressure and electric power for the generation of a low temperature plasma is supplied to electrodes surrounding the tubular body in the insulating oil. Several of the problems in the prior art methods can be solved by the present invention such as undesirable thermal degradation of the tube surface by the evolved by the electric discharge, and power loss due to the generation of plasma outside the tubular body. When the method of the invention is applied to the treatment of a continuous length tubular body, the tubular body is continuously moved in the insulating oil through the electrodes surrounding it, with the optional provision of a sleeve made of an insulating material placed between the electrodes and the tubular body, so as that the tubular body is free from adverse effects caused by direct contact with the electrodes.