A seating sensor comprising: a holder configured to be placed, in a state of being sandwiched between a cushion member of a seat cushion of a vehicle and a support member that supports the cushion member, on the support member; a sensor body placed on the holder and configured to detect information of a load applied from the cushion member; a fitting portion including a first fitting piece protruding downward from an end portion of the holder and a second fitting piece protruding downward from the holder in the vicinity to the first fitting piece behind the first fitting piece, and mountable on the support member in a state where the support member is fitted between the first and second fitting pieces (34a and 34b); a slip-off preventing portion protruding from a tip end of the first fitting piece toward the second fitting piece; an arm portion protruding downward from the holder; and a second slip-off preventing portion protruding from a tip end of the arm portion and configured to be brought into contact with or located close to the support member from an upper side, characterized in that the seating sensor further comprises: a pair of the arm portions that are arranged side by side in a first direction orthogonal to a direction in which the first and second fitting pieces (34a and 34b) are arranged side by side and configured to be brought into contact with or located close to the support member at one side and the other side in the first direction; and a backlash suppressing portion (38, 50) protruding downward from the holder and located close to the support member in the first direction. a holder configured to be placed, in a state of being sandwiched between a cushion member of a seat cushion of a vehicle and a support member that supports the cushion member, on the support member; a sensor body placed on the holder and configured to detect information of a load applied from the cushion member; a fitting portion including a first fitting piece protruding downward from an end portion of the holder and a second fitting piece protruding downward from the holder in the vicinity to the first fitting piece behind the first fitting piece, and mountable on the support member in a state where the support member is fitted between the first and second fitting pieces (34a and 34b); a slip-off preventing portion protruding from a tip end of the first fitting piece toward the second fitting piece; an arm portion protruding downward from the holder; and a second slip-off preventing portion protruding from a tip end of the arm portion and configured to be brought into contact with or located close to the support member from an upper side, characterized in that the seating sensor further comprises: a pair of the arm portions that are arranged side by side in a first direction orthogonal to a direction in which the first and second fitting pieces (34a and 34b) are arranged side by side and configured to be brought into contact with or located close to the support member at one side and the other side in the first direction; and a backlash suppressing portion (38, 50) protruding downward from the holder and located close to the support member in the first direction. a pair of the arm portions that are arranged side by side in a first direction orthogonal to a direction in which the first and second fitting pieces (34a and 34b) are arranged side by side and configured to be brought into contact with or located close to the support member at one side and the other side in the first direction; and a backlash suppressing portion (38, 50) protruding downward from the holder and located close to the support member in the first direction.