A needle assembly for transdermal administration, comprising a substrate including a first surface and a second surface opposite to the first surface, and a plurality of fine needles projecting perpendicularly from the first surface, wherein the substrate includes a plurality of grooves in at least one of the first surface and the second surface, and the plurality of grooves are formed to allow the substrate to deform following a surface shape of a skin of a body, such that axes of the fine needles are each displaced in a direction coinciding with a direction of a line normal to the surface of the skin of the body, wherein the fine needles are arranged in a matrix in the first surface of the substrate at regular intervals along a longitudinal direction and a lateral direction of the first surface, the plurality of grooves include a plurality of grooves each formed in the first surface so as to extend in a first direction between the fine needles adjacent to each other, and to be arranged along a second direction perpendicular to the first direction, and the first direction includes at least one of the longitudinal direction, the lateral direction, and a diagonal direction of the first surface, characterized in that the plurality of grooves further includes a plurality of grooves each formed in the second surface so as to extend in a third direction and to be arranged along a fourth direction perpendicular to the third direction, wherein the third direction includes at least one of a longitudinal direction, a lateral direction, and a diagonal direction of the second surface, and wherein positions of the grooves in the second surface are offset from positions of the grooves in the first surface as seen perpendicular to the first surface. the substrate includes a plurality of grooves in at least one of the first surface and the second surface, and the plurality of grooves are formed to allow the substrate to deform following a surface shape of a skin of a body, such that axes of the fine needles are each displaced in a direction coinciding with a direction of a line normal to the surface of the skin of the body, wherein the fine needles are arranged in a matrix in the first surface of the substrate at regular intervals along a longitudinal direction and a lateral direction of the first surface, the plurality of grooves include a plurality of grooves each formed in the first surface so as to extend in a first direction between the fine needles adjacent to each other, and to be arranged along a second direction perpendicular to the first direction, and the first direction includes at least one of the longitudinal direction, the lateral direction, and a diagonal direction of the first surface, characterized in that the plurality of grooves further includes a plurality of grooves each formed in the second surface so as to extend in a third direction and to be arranged along a fourth direction perpendicular to the third direction, wherein the third direction includes at least one of a longitudinal direction, a lateral direction, and a diagonal direction of the second surface, and wherein positions of the grooves in the second surface are offset from positions of the grooves in the first surface as seen perpendicular to the first surface.