In the image signal processing apparatus in this invention, control is carried out in such a manner that an enlarged image signal based on an image signal stored in a memory is produced a predetermined period of time after the image signal was written into the memory, a blanking signal is generated at a predetermined period of time after a further blanking signal included in the image signal applied into the memory occurs, so that during the blanking period occurring in the image signal, the blanking signal is produced, and during periods outside this blanking period, the output is changed over for the enlarged image signal produced from the memory so that the image signal may be enlarged in a stable manner, and the construction and work involved in circuit adjustment may be simple.