Abstract: A memory-sharing system device has a shared memory, divided into forward-direction and backward-direction memory areas; a first processor inputting transfer data in the forward direction, writing the data to the forward-direction memory area, reading transfer data in the backward direction from the backward-direction memory area and outputting the data; and a second processor for transferring data in the back-ward direction. The first or second processor sets memory release criteria for the forward-direction and backward-direction memory areas respectively, and, when the used memory area reaches the memory release criterion, performs memory release processing. The first or second processor monitors the forward-direction and the backward-direction data transfer speed, changes the memory release criterion depending on the data transfer speed.