An attachment tape is configured such that a light emitter and a light detector are mounted on the attachment tape and such that the attachment tape is wrapped around a tissue so as to be attached to the tissue. The light emitter is configured to emit light. The light detector is configured to detect the light emitted from the light emitter. A width dimension of the attachment tape is gradually narrowed from one point along a longitudinal direction of the attachment tape toward another point at a distal end of the attachment tape.
