We propose a semiconductor directional-coupler-type all-optical switch with tapered structures which is based on the nonlinear refractive index variation induced by photoexcited carriers. A recurrence equation describing the device is derived from `Marcatili's' coupling mode theory and studied by numerical simulations. It is shown that the introduction of tapered structures is effective for improving the extinction ratio. It is also found that this device can drive a signal light at a power about 17 dB higher than the control light power.