PURPOSE: To obtain a strong bubble stretching capability and a stable bubble transfer characteristic by adopting an asymmetrical structure to the inlet side element chip of a chevron pattern shorter than an exit side element chip.

CONSTITUTION: The asymmetrical structure is adopted in which the length l1 of the inlet side element chip 11 of the chevron pattern 10 is shorter than a length l2 of the exit side element chip 12. The length of the inlet side element chip 11 of the chevron pattern 10 where the pattern width W1 at peaks is wider than the pattern width W2 at troughs is made shorter than the length of the exist side element chip 12 in this way, then the shape anisotropy of the inlet side element chip 11 is not increased to prevent the deterioration in the bubble transfer characteristic at the inlet side element chip. Moreover, a deep potential is formed at the tip of the exit side element chip 12 is formed to increase the bubble expansion capability.