We here report a novel optical second harmonic generation (SHG) measurement that allows an electric field formed in organic solid to be probed. We examined the SHG intensity profile that changes depending on a space charge field caused by carrier injection. Experiments making use of time-resolved SHG technique has revealed dynamic changes of SHG intensity profiles arising from pentacene, and that carrier transport in OFET was diffusion-like. Calculations using drift-diffusion equation well accounted for the visualized carrier motion probed by time-resolved SHG. That is, interface charge propagation process dominates the carrier transport, which is regulated by applied gate-source voltage. (C) 2010 Elsevier B.V. All rights reserved.