A test pattern preparation system for testing an LSI circuit, the system includes: a circuit data file for storing various circuit data; an old test pattern file for storing old test patterns; a test pattern preparation unit for performing a logic simulation, detecting "simultaneous-change action" based on a result of the logic simulation, and preparing a new test pattern in accordance with the circuit data and the old test patterns; and a new test pattern file for storing new test patterns which are prepared by the test pattern preparation unit.