The lengthening of ferrite plates in a high-strength low carbon low alloy steel, which contained 1.62 mass% Mn and small amounts of Si, Mo, Ni, Cr, and Nb, was observed in situ under high-temperature confocal laser scanning microscopy. Applying the analysis by Speich and Cohen to the growth of a lath-type plate, it was found that the measured lengthening rates are likely to fall between those calculated assuming para- and NPLE modes from the semiempirical equation proposed by Hillert, while they scattered widely presumably because the plate morphology was not a lenticular disk. In the temperature range of measurement, solute drag due to alloying element segregation at the boundary between plate tip and the matrix may not operate to an appreciable extent. The retardation of lengthening from that calculated under paraequilibrium could occur by intrinsic friction of alpha/gamma boundary.