The invention relates to a case structure for a sensor structure attachable to and detachable from a shoe, the case structure comprising a case and an attachment arrangement for attaching the case to the shoe, the attachment arrangement comprising an attachment projection structure intended to attach the case to the shoe such that shoelaces remain between a bottom comprised by the case and the attachment projection structure below the bottom. The essential feature in the invention is that the attachment projection structure is a branched projection structure comprising a first projection branch and a second projection branch, and, between the projection branches, an intermediate area intended to enable the placing of the case structure in position even if the shoelaces were arranged to support a tongue of the shoe.