[Object] To provide a full float type crankshaft device for bicycle which allows a sub-assembly formed by integrating crankshafts to be mounted on a body side.[Constitution] The crankshaft device includes a shaft 1 provided at a center of a cylindrical hanger member 9, having cranks attached to both end portions, which is configured to have bearings 3,3 provided at inner sides where the cranks are attached, a housing 2 configured at an outer diameter side of the shaft 1 to support the shaft 1 via the bearings 3, and clamp nuts 5 each with a cup-like shape as a whole, which are attached to both end portions of the housing 2 having O-rings 6 as rubber elastic elements interposed between both end portions.