The present invention provides a mini ultrasonic probe which comprises an acoustic backing layer with high strength, high insulating property, high interchannel homogeneity, excellent exothermic character and big damping decrement. The acoustic backing layer is characterized in that: the ultrasonic probe has an acoustic backing layer made of a composite resin material including a resin and a plurality of bonded fibers contained the resin, and the composite resin material exhibits an acoustic impedance of 1.3 to 6 MRayls at 25 DEG C. The bonded fibers is formed of a plurality of zinc oxide fibers bonded to each other at one edge portions and extending in different directions in the other edge portion; a plurality of channels are arranged on the acoustic backing layer with space, and each channel has a piezoelectric element and an acoustic matching layer formed on the piezoelectric element; and an acoustic lens formed to cover at least the surface of the acoustic matching layer of each channel.