A portable stress-applying device for stress radiography was developed for daily clinical use. Using this device, stress radiography for the diagnosis of the anterior cruciate ligament (ACL) deficiency was performed with the knee flexed at 20 ° and at 90 °. A 100-N force was chosen as a standardized stress. The subjects were classified into four groups: the manually tested ACL-deficient group (32 knees), the manually tested control group (80 knees), the instrumentally tested ACL-deficient group (14 knees), and the instrumentally tested control group (34 knees). There was no statistical difference in the reliability (sensitivity, specificity, and accuracy) of stress radiography between the manual technique and the instrumental technique. When stress radiography with the knee flexed at 20 ° and that at 90 ° were compared, the former was more reliable than the latter. As the manual technique is compromised by a lack of standardization in applied force, a mechanical device is required in quantitative stress radiography. The reliability of stress radiography with the knee flexed at 20 ° is considered high enough to warrant dispensing with further stress radiography with the knee flexed at 90 ° for diagnosing ACL deficiency.