A standing motion assist system includes a care belt including a holding mechanism including a holder that holds a back and both armpits of a care-receiver, and a connector that is connected to the holding mechanism; a rotational force applying mechanism that is connected to the holding mechanism and that rotates a front lower part of the holder upward and a rear upper part of the holder downward about a rotation axis extending through both armpits; a traction mechanism that is connected to the connector and pulls the connector; and a controller that controls the rotational force applying mechanism and the traction mechanism so that, after the traction mechanism has started the pulling motion, the rotational force applying mechanism rotates the holder at the same time as the traction mechanism pulls the connector forward and upward, and subsequently the traction mechanism pulls the connector upward.