This invention provides an ultrasonic phantom that can respond to different ultrasonic intensities. Such ultrasonic phantom comprises a gel comprising a low-boiling compound in the form of droplets sealed therein. The invention also provides an assay technique using such phantom. This invention enables regulation of components of droplets, droplet size, and the ultrasonic intensity at which droplets are vaporized upon mixing of droplets with different sizes. This enables visualization of whether or not ultrasound beams that exceed relevant different ultrasonic intensities have been applied.