Provided is a vehicle-body lower structure with a side collision sensor which vehicle-body lower structure can attain early sensing by improving a body structure. A first load transmission member of which a longitudinal section has a generally Z-shape is disposed in a closed section of a rocker. A first joining portion of first load transmission member is joined in a state where first joining portion is sandwiched between an upper flange of a rocker outer panel and an upper flange of a rocker inner panel, and a second joining portion is joined to an intermediate part, in a height direction, of a vertical wall portion of rocker outer panel. A body portion connects first joining portion and second joining portion diagonally, and transmits a load input into rocker outer panel at time of a side collision, to an upper-wall-portion side of rocker inner panel by a shortest distance.