In-plane bending vibration mode is a good candidate for pipe inspection. First, using finite element method the characteristics of in-plane bending mode are investigated. It is confirmed that in-plane bending mode has small dependence on the pipe length and the boundary conditions at the pipe ends, it appears in the audio frequency, and it has linear dependence on the pipe thickness. In addition, formula with two dimensional ring approximation treating as in-plane bending of curved beam is derived for parameter identification. Moreover, in-plane bending mode is observed experimentally on the actual pipe system and its resonant frequency shows good match with the ring approximation. For application to actual pipe inspection, the ring approximation is compared with finite element model which has distributed deteriorated regions.