Factor oracles constructed 
from a given text are deterministic acyclic automata 
accepting all substrings of the text. Factor oracles are more space 
economical and easy to implement than similar data structures 
such as suffix tree. 
There is, however, some drawback; 
a factor oracle may accept strings not in the text, 
which we call a {\em error acceptance}. 
In this paper, we charactrize factor orales that 
accept nonsubstrings erroreously. 
Using this characterization, 
we propose an algorithm to decide 
whether a factor oracle makes an error acceptance 
during its linear time construction.