An in-tire electric device that includes a bracket fixed to an outer peripheral surface of a rim of a tire wheel by a belt wound around the outer peripheral surface of the rim and an electric circuit unit detachably held by the bracket, the in-tire electric device comprising: an engagement protrusion (61, 62) and an engagement hole (26, 28) that are provided on the electric circuit unit and the bracket and engage with each other in such a way as to restrict, through a two-step operation in which the electric circuit unit is moved in a first direction with respect to the bracket and then moved or rotated in a second direction intersecting the first direction, all movements and rotations except for a reverse of the second direction; a retainer that is mounted on the bracket from a direction intersecting the second direction and faces the electric circuit unit in the second direction to restrict the electric circuit unit from moving in the second direction; and retainer locking portions (72, 23) that are formed on the bracket and the retainer and engage with each other by elastic deformation of one of the retainer locking portions (72, 23) to lock the retainer to the bracket, characterized in that the engagement protrusion (61, 62) is constituted by a pair of engagement protrusions (61, 62) that protrude from the electric circuit unit toward the outer peripheral surface of the rim, and have foot portions enlarged at tip portions with respect to base end portions in a third direction that is perpendicular to both the first direction in which the engagement protrusions (61, 62) protrude and the second direction in which the engagement protrusions (61, 62) face each other, the bracket is provided with a pair of pedestal protrusions that protrude toward a side away from the outer peripheral surface of the rim and have, at tip portions, upper surface walls that are perpendicular to a direction in which the pair of pedestal protrusions protrude, the engagement hole (26, 28) is constituted by a pair of engagement holes that pass through the upper surface walls of the pair of pedestal protrusions, the pair of engagement holes (26, 28) are provided with wide portions suitable for the foot portions of the pair of engagement protrusions (61, 62) to pass through and narrow portions that are located on one side of the wide portions in the second direction and suitable for the base end portions of the pair of engagement protrusions (61, 62) to pass through, and the electric circuit unit is moved in the first direction, the pair of engagement protrusions (61, 62) are inserted into the wide portions of the pair of engagement holes (26, 28) and then moved in the second direction, the base end portions of the pair of engagement protrusions (61, 62) are engaged with the narrow portions of the pair of engagement holes (26, 28), and then the retainer is inserted into the wide portion of any of the pair of engagement holes (26, 28). an engagement protrusion (61, 62) and an engagement hole (26, 28) that are provided on the electric circuit unit and the bracket and engage with each other in such a way as to restrict, through a two-step operation in which the electric circuit unit is moved in a first direction with respect to the bracket and then moved or rotated in a second direction intersecting the first direction, all movements and rotations except for a reverse of the second direction; a retainer that is mounted on the bracket from a direction intersecting the second direction and faces the electric circuit unit in the second direction to restrict the electric circuit unit from moving in the second direction; and retainer locking portions (72, 23) that are formed on the bracket and the retainer and engage with each other by elastic deformation of one of the retainer locking portions (72, 23) to lock the retainer to the bracket, characterized in that the engagement protrusion (61, 62) is constituted by a pair of engagement protrusions (61, 62) that protrude from the electric circuit unit toward the outer peripheral surface of the rim, and have foot portions enlarged at tip portions with respect to base end portions in a third direction that is perpendicular to both the first direction in which the engagement protrusions (61, 62) protrude and the second direction in which the engagement protrusions (61, 62) face each other, the bracket is provided with a pair of pedestal protrusions that protrude toward a side away from the outer peripheral surface of the rim and have, at tip portions, upper surface walls that are perpendicular to a direction in which the pair of pedestal protrusions protrude, the engagement hole (26, 28) is constituted by a pair of engagement holes that pass through the upper surface walls of the pair of pedestal protrusions, the pair of engagement holes (26, 28) are provided with wide portions suitable for the foot portions of the pair of engagement protrusions (61, 62) to pass through and narrow portions that are located on one side of the wide portions in the second direction and suitable for the base end portions of the pair of engagement protrusions (61, 62) to pass through, and the electric circuit unit is moved in the first direction, the pair of engagement protrusions (61, 62) are inserted into the wide portions of the pair of engagement holes (26, 28) and then moved in the second direction, the base end portions of the pair of engagement protrusions (61, 62) are engaged with the narrow portions of the pair of engagement holes (26, 28), and then the retainer is inserted into the wide portion of any of the pair of engagement holes (26, 28).