Abstract: Inertia sensor in belt reels for vehicle belts comprising a rolling body (12) carried on a support (10), and a transmission element (14) for operative interaction with the latch mechanism of the belt reel. This transmission element comprises a double-arm lever (16, 18) which surrounds the rolling body and is resiliently biased, so that one arm (16) will engage the lower side of the rolling body, the transmission element thus being adjustable under the bias thereof at movement of the rolling body from a rest position on the support. The other arm (18) normally is spaced from the upper side of the rolling body and is arranged to actuate the latch mechanism by such adjustment. At delayed movement of the transmission element under the bias thereof the rolling body will engage said other arm to actuate the latch mechanism.