PURPOSE: To reduce the development cost of a program and to expand versatility by permitting a user program to directly access to file space including a file on a sequential access medium.

CONSTITUTION: The device consist of a file mapping means 12 executing mapping with the file space of logical space and a file access means 13 accessing to the file by a logical address. A file management mechanism opens the original file to the file open request of the user program and reserves logical space equivalent to the information amount of the file. The file which is opened in logical space is set in a temporary assignment state (real memory is not assigned and page information is in a state where it is left on a secondary storage medium as it is). The head logical address which is temporarily assigned is returned to the user program as the base address of the file. Thus, the user program accesses to the file with the address as a base address.