1. A flash memory system comprising: a flash memory including a storage area divided into a plurality of storage blocks each for storing data; means for storing degradation degrees of individual ones of said storage blocks; control means for determining, when a data writing operation is requested, a storage block into which requested write data are to be written, referring to said degradation degrees stored in said means for storing the degradation degrees; means for writing the write data into the determined storage block, and measuring a length of time spent to write the data into the determined storage block; and means for diagnosing a degradation degree of the determined storage block based on the measured length of time spent to write the data into the determined storage block, and writing the diagnosed degradation degree of the determined storage block in said means for storing the degradation degrees.