According to an embodiment, an encryption key generating apparatus includes a converting unit to convert input data using a physically unclonable function and outputs output data; a memory to store a plurality of pattern data, each of which is a partial data in the output data indicated by one of a plurality of index data; a generating unit to generates an encryption key on the basis of the plural of index data; and a comparing unit to compare the output data with the plural of pattern data to detect plural of locations in the output data at which partial data similar to the plural of pattern data is present. The generating unit reproduces, as the plural of index data, the plural of locations detected by the comparing unit and reproduces the encryption key on the basis of the plural of index data that have been reproduced.