A vehicle-body lower structure with a side collision sensor, wherein the vehicle-body lower structure comprises: a rocker placed at an outer side of a vehicle-body lower portion in a vehicle width direction so as to extend along a vehicle front-rear direction, the rocker having a closed-section structure constituted by a rocker outer panel and a rocker inner panel such that an upper flange formed in an upper part of the rocker outer panel is joined to an upper flange formed in an upper part of the rocker inner panel; a side collision detection sensor attached to the rocker inner panel or a member joined to the rocker inner panel, or a lower part of a pillar provided in a standing manner from the rocker, so as to detect a side collision based on an acceleration; characterized in that said vehicle-body structure further comprises: a first load transmission member placed in a closed section of the rocker so as to partition the closed section longitudinally, the first load transmission member being configured such that a first joining portion of the first load transmission member is joined in a state where the first joining portion is sandwiched between the upper flange of the rocker inner panel or the lower part of the pillar and the upper flange of the rocker outer panel and a second joining portion of the first load transmission member is joined to a vertical wall portion placed at an outer side of the rocker outer panel in the vehicle width direction, the vertical wall portion extending in a vehicle up and down direction. a rocker placed at an outer side of a vehicle-body lower portion in a vehicle width direction so as to extend along a vehicle front-rear direction, the rocker having a closed-section structure constituted by a rocker outer panel and a rocker inner panel such that an upper flange formed in an upper part of the rocker outer panel is joined to an upper flange formed in an upper part of the rocker inner panel; a side collision detection sensor attached to the rocker inner panel or a member joined to the rocker inner panel, or a lower part of a pillar provided in a standing manner from the rocker, so as to detect a side collision based on an acceleration; characterized in that said vehicle-body structure further comprises: a first load transmission member placed in a closed section of the rocker so as to partition the closed section longitudinally, the first load transmission member being configured such that a first joining portion of the first load transmission member is joined in a state where the first joining portion is sandwiched between the upper flange of the rocker inner panel or the lower part of the pillar and the upper flange of the rocker outer panel and a second joining portion of the first load transmission member is joined to a vertical wall portion placed at an outer side of the rocker outer panel in the vehicle width direction, the vertical wall portion extending in a vehicle up and down direction. a first load transmission member placed in a closed section of the rocker so as to partition the closed section longitudinally, the first load transmission member being configured such that a first joining portion of the first load transmission member is joined in a state where the first joining portion is sandwiched between the upper flange of the rocker inner panel or the lower part of the pillar and the upper flange of the rocker outer panel and a second joining portion of the first load transmission member is joined to a vertical wall portion placed at an outer side of the rocker outer panel in the vehicle width direction, the vertical wall portion extending in a vehicle up and down direction.