This paper reports a method to focus sound from a moving source in liquid to a fixed acoustic sensor using a varifocal acoustic mirror made from silicone rubber. A boundary between silicone rubber and air works as an acoustic mirror whose curvature radius can be changed. Simulation results showed that the spherical surface was able to focus sound coming from a point source to a certain point. It was confirmed that a fabricated varifocal acoustic mirror improved the ouptut of an acoustic sensor for different sound source positions. The curvature radius of the acoustic mirror corresponding to the maximum sensor output conformed to the value calculated using a theoretical equation. The sound pressure where the sensor was positioned became at most 12 times larger than that without the acoustic mirror.