In this work, we apply a virtual-focus scanning method to detect a tumor inside a simplified two-dimensional breast model via the finite difference time domain method. For the purpose of simplification, we construct a microwave detection system with an N-element infinitesimal dipole array, and excite the central element of the array while letting all elements receive the time-shifted scattered fields in order to filter out unwanted signals. With the application of the two-step, virtual-focus scanning method, our computerized prototype system has accurately detected randomly generated tumors in all cases investigated, including small tumors in early stages of development.