PURPOSE: To make an assembling easy by providing a horizontal direction passage in an axial direction of a hub, having a shoulder extending in the axial direction at each axial direction tip part of the passage by the hub and having the inside tip part passing through the passage and a notch storing the shoulder so as to be engaged with the spring in the passage.

CONSTITUTION: To assemble a rule, a hub 22 is stored in a reel part 24 and an inside tip part 88 of a spring 86 is inserted into a passage 80 of the hub 22. When the spring 86 is penetrated into the hub 22, a shoulder 84 with gradient bends a tip edge part of the spring 86 upward so that the tip edge part may pass on the shoulder 84 till a notch 90 is matched with the shoulder 84 and when matched, the tip edge part repels downwards beyond the shoulder 84. The tip edge part fixes the spring 86 to the shoulder 84 within the passage 80 and subsequently the projecting tip part is bent. The shoulder 84 and the notch 90 ensure a proper location of the spring 86 for the hub 22 and perform a fixing engagement between the hub 22 and the spring 86.