  A magnetic domain wall (DW) behaves as a massive particle with elasticity.
Sliding and oscillation of the DW have been observed experimentally, whereas
vibration of a width in the DW, "breathing mode", has not been measured so far.
We theoretically propose how to observe the breathing mode by the Josephson
junction having a ferromagnetic layer between superconducting electrodes. The
current-voltage (I-V) curve is calculated by an equivalent circuit of the
resistively shunted junction model. The breathing mode is identified by
stepwise structures in the I-V curve, which appear at the voltages
V=n(hbar/2e)omega with the fundamental constant hbar/e, integer number n, and
the frequency of the breathing mode omega.
