A band device including: an annular band portion that forms a hollow portion, wherein the band portion includes: a sensor module; end pieces (20; 20A, 20B) attachable to and detachable from the band portion; and an engaging portion detachably engaged with the end pieces (20; 20A, 20B) to which a watch head is attached, characterized in that the end pieces each (20; 20A, 20B) include a piece housing portion including a back surface located on a wearer's skin side and a front surface on a side opposite to the back surface, and a connecting portion protruding from the back surface of the piece housing portion to a side of the band portion, and the engaging portion includes an opening into which the connecting portion of the end piece (20; 20A; 20B) is inserted, wherein the connecting portion includes a shaft portion formed along a thickness direction of the piece housing portion, and an overhang portion overhanging from an end portion of the shaft portion in a direction perpendicular to the thickness direction, and the engaging portion includes abutment surfaces that abut the overhang portion on a back surface of the band portion, wherein the end pieces (20;20A;20B) each include an elastic portion that urges the connecting portion to a side of the front surface, wherein the shaft portion is provided rotatably with respect to the piece housing portion. an annular band portion that forms a hollow portion, wherein the band portion includes: a sensor module; end pieces (20; 20A, 20B) attachable to and detachable from the band portion; and an engaging portion detachably engaged with the end pieces (20; 20A, 20B) to which a watch head is attached, characterized in that the end pieces each (20; 20A, 20B) include a piece housing portion including a back surface located on a wearer's skin side and a front surface on a side opposite to the back surface, and a connecting portion protruding from the back surface of the piece housing portion to a side of the band portion, and the engaging portion includes an opening into which the connecting portion of the end piece (20; 20A; 20B) is inserted, wherein the connecting portion includes a shaft portion formed along a thickness direction of the piece housing portion, and an overhang portion overhanging from an end portion of the shaft portion in a direction perpendicular to the thickness direction, and the engaging portion includes abutment surfaces that abut the overhang portion on a back surface of the band portion, wherein the end pieces (20;20A;20B) each include an elastic portion that urges the connecting portion to a side of the front surface, wherein the shaft portion is provided rotatably with respect to the piece housing portion. a sensor module; end pieces (20; 20A, 20B) attachable to and detachable from the band portion; and an engaging portion detachably engaged with the end pieces (20; 20A, 20B) to which a watch head is attached, characterized in that the end pieces each (20; 20A, 20B) include a piece housing portion including a back surface located on a wearer's skin side and a front surface on a side opposite to the back surface, and a connecting portion protruding from the back surface of the piece housing portion to a side of the band portion, and the engaging portion includes an opening into which the connecting portion of the end piece (20; 20A; 20B) is inserted, wherein the connecting portion includes a shaft portion formed along a thickness direction of the piece housing portion, and an overhang portion overhanging from an end portion of the shaft portion in a direction perpendicular to the thickness direction, and the engaging portion includes abutment surfaces that abut the overhang portion on a back surface of the band portion, wherein the end pieces (20;20A;20B) each include an elastic portion that urges the connecting portion to a side of the front surface, wherein the shaft portion is provided rotatably with respect to the piece housing portion.