An off-axis sawtooth oscillation is excited by electron cyclotron heating in a lower hybrid current drive plasma in the WT-3 tokamak. The internal structure of the plasma is investigated using computer tomography with five detector arrays after the application of singular value decomposition to the soft X-ray detector signals. It is found that the precursor oscillation of the off-axis sawtooth collapse is the m = 2 mode. On the other hand, the double m = 2 mode structure is observed as a postcursor of the collapse.