A seating sensor comprising: a holder (31, 50) including a first fall-off preventing portion (33, 51) and a first positioning portion (34, 52), and being mountable on a seat cushion of a vehicle; and a sensor body (40, 55) mounted on the holder (31, 50) and including a second fall-off preventing portion (44, 56) engaged with the first fall-off preventing portion (33, 51) and configured to restrain fall-off of the sensor body (40, 55) from the holder (31, 50) in a predetermined fall-off preventing direction and a second positioning portion (44a, 56a) engaged with the first positioning portion (34, 52) and configured to restrain a positional deviation of the sensor body (40, 55) relative to the holder (31, 50) in a direction intersecting with the fall-off preventing direction, the sensor body (40, 55) being configured to detect information of a load applied to the seat cushion, wherein the first fall-off preventing portion (33, 51) is a base portion and a lid portion which are provided in parallel in the fall-off preventing direction, the second fall-off preventing portion (44, 56) is an interposing portion sandwiched between the base portion and the lid portion in the fall-off preventing direction, the first positioning portion (34, 52) is a positioning projection provided to protrude from any one of the base portion and the lid portion in the fall-off preventing direction, the second positioning portion (44a, 56a) is a positioning recess which is formed through the interposing portion in the fall-off preventing direction to face the positioning projection and into which the positioning projection is inserted, the other one of the base portion and the lid portion, which is not provided with the positioning projection, has a relief hole formed therein, into which a tip end portion of the positioning projection penetrating the positioning recess is inserted, the base portion is fixedly provided on the holder (31, 50), and the seating sensor further comprises a fitting claw provided to protrude from one of the base portion and the lid portion, and a fitting receiving portion provided on the other one of the base portion and the lid portion and having a fitting hole formed therein so as to enable the fitting claw to be fitted thereto at a rotational position where the base portion and the lid portion are arranged in parallel in the fall-off preventing direction, characterized in that the lid portion is rotatably connected to the base portion via a flexible portion, the base portion and the lid portion are integrally provided in the holder (31, 50). a holder (31, 50) including a first fall-off preventing portion (33, 51) and a first positioning portion (34, 52), and being mountable on a seat cushion of a vehicle; and a sensor body (40, 55) mounted on the holder (31, 50) and including a second fall-off preventing portion (44, 56) engaged with the first fall-off preventing portion (33, 51) and configured to restrain fall-off of the sensor body (40, 55) from the holder (31, 50) in a predetermined fall-off preventing direction and a second positioning portion (44a, 56a) engaged with the first positioning portion (34, 52) and configured to restrain a positional deviation of the sensor body (40, 55) relative to the holder (31, 50) in a direction intersecting with the fall-off preventing direction, the sensor body (40, 55) being configured to detect information of a load applied to the seat cushion, wherein the first fall-off preventing portion (33, 51) is a base portion and a lid portion which are provided in parallel in the fall-off preventing direction, the second fall-off preventing portion (44, 56) is an interposing portion sandwiched between the base portion and the lid portion in the fall-off preventing direction, the first positioning portion (34, 52) is a positioning projection provided to protrude from any one of the base portion and the lid portion in the fall-off preventing direction, the second positioning portion (44a, 56a) is a positioning recess which is formed through the interposing portion in the fall-off preventing direction to face the positioning projection and into which the positioning projection is inserted, the other one of the base portion and the lid portion, which is not provided with the positioning projection, has a relief hole formed therein, into which a tip end portion of the positioning projection penetrating the positioning recess is inserted, the base portion is fixedly provided on the holder (31, 50), and the seating sensor further comprises a fitting claw provided to protrude from one of the base portion and the lid portion, and a fitting receiving portion provided on the other one of the base portion and the lid portion and having a fitting hole formed therein so as to enable the fitting claw to be fitted thereto at a rotational position where the base portion and the lid portion are arranged in parallel in the fall-off preventing direction, a fitting claw provided to protrude from one of the base portion and the lid portion, and a fitting receiving portion provided on the other one of the base portion and the lid portion and having a fitting hole formed therein so as to enable the fitting claw to be fitted thereto at a rotational position where the base portion and the lid portion are arranged in parallel in the fall-off preventing direction, characterized in that the lid portion is rotatably connected to the base portion via a flexible portion, the base portion and the lid portion are integrally provided in the holder (31, 50).