To inexpensively provide a washer member that provides highly reliable fastening performance without a variation even if a worker varies, by surely applying fastening torque required when fastening a fastening object member by simple fastening work, while using an ordinary tool, without requiring the tool to be set to predetermined fastening torque before use like a tool having an idly rotating function, when assembling a lightweight steel frame such as prefabrication using a washer member.

This washer member includes integrally a nut part 2 and a washer part 3 with a diameter enlarged more than an external shape of the nut part 2. A groove part 4 having the predetermined depth on an under surface of a washer 3 is formed in a boundary part between the nut part 2 and the washer part 3. A connecting part 5 is formed between the groove part 4 and a screw forming part. When fastening the fastening object member in cooperation with a bolt member, the connecting part 5 ruptures by the predetermined fastening torque.