PURPOSE: To make it hard to estimate check items and also make it difficult to forge a paper sheet by providing a decision means which decides the paper sheet according to check items selected on the basis of random data.

CONSTITUTION: A random data generation, circuit 1 is equipped with a random number generator and outputs different random data to a discrimination data selecting circuit 2 each time a specific number of paper sheets are received by a paper sheet processing machine. The discrimination data selecting circuit 2 selects several check items among all items that the discriminator can check according to the respective random data. A decision circuit 3 consisting of a CPU extracts data through a discrimination data extraction circuit 4 consisting of various sensors as to only the selected check items and performs arithmetic processing on the basis of the extracted data to decide whether or not the paper sheets are genuine according to whether or not specific conditions are met.