Shock waves have recently attracted attention for their effects on biological soft tissues, such as angiogenesis. When shock waves are reflected at some surfaces of biological tissues, an expansion wave with as short duration as shock wave can be generated because of a change in acoustic impedance. We hypothesized that the expansion wave affects soft tissues, inducing bioeffects. In this study, we developed an expansion wave generator consisting of an ellipsoidal reflector and a polydimethyl siloxane (PDMS) unit. A shock wave (compression wave) was generated by underwater electric discharge and focused by reflection from an ellipsoidal mirror. The shock wave propagated in the water and was transmitted at the water-PDMS boundary surface because they have almost the same acoustic impedance (1.5× 106 kg/m2･ s for water, approximately 1.4× 106 kg/m2･ s for PDMS). Next, the shock wave was reflected at the PDMS-air boundary surface. At this surface, the shock wave was converted to an expansion wave because the acoustic impedance of PDMS is much higher than that of air (4.3× 102 kg/m2･ s). After these reflections, the expansion wave was focused. At the focal point, the peak pressure measured by a hydrophone was− 5.52±1.23 MPa (n= 10) when the discharge voltage was 4 kV. Moreover, the half-width of the expansion wave was 0.55±0.16 μs (n= 10). These values were the same as those of the shock wave. These results indicate that our expansion wave generator is able to produce expansion waves and is useful for studying their bioeffects.