A multi-antenna device comprising: a grounding plate; a first antenna including a first feed element that is grounded to the grounding plate via a first feed point; and a second antenna including a second feed element that is grounded to the grounding plate via a second feed point, wherein the grounding plate is formed in a shape that is substantially in point symmetry relative to the center of the grounding plate, or the grounding plate has a polyhedral shape in the plan view, and the first feed point and the second feed point are disposed near vertices of the grounding plate, respectively, wherein the first feed point and the second feed point are disposed such that a straight line connecting the first feed point and a center of the grounding plate and a straight line connecting the second feed point and the center of the grounding plate are substantially perpendicular to each other in a plan view; characterized in that the first antenna and the second antenna further include a first passive element and a second passive element, respectively, the first passive element and the second passive element each having a plurality of folded-back parts (121, 122, 123, 124, 125) that are folded back at a plurality of locations; the first feed element and the second feed element each have a first portion and a second portion that is wider than the first portion; the second portions of the first feed element and the second feed element have widths that are greater than widths of the first passive element and the second passive element in a direction perpendicular to a direction in which the folded-back parts (121, 122, 123, 124, 125) extend, respectively; and at least the second portions of the first feed element and the second feed element are configured to couple to the folded-back parts (121, 122, 123, 124, 125) of the first passive element and the second passive element, respectively. a grounding plate; a first antenna including a first feed element that is grounded to the grounding plate via a first feed point; and a second antenna including a second feed element that is grounded to the grounding plate via a second feed point, wherein the grounding plate is formed in a shape that is substantially in point symmetry relative to the center of the grounding plate, or the grounding plate has a polyhedral shape in the plan view, and the first feed point and the second feed point are disposed near vertices of the grounding plate, respectively, wherein the first feed point and the second feed point are disposed such that a straight line connecting the first feed point and a center of the grounding plate and a straight line connecting the second feed point and the center of the grounding plate are substantially perpendicular to each other in a plan view; characterized in that the first antenna and the second antenna further include a first passive element and a second passive element, respectively, the first passive element and the second passive element each having a plurality of folded-back parts (121, 122, 123, 124, 125) that are folded back at a plurality of locations; the first feed element and the second feed element each have a first portion and a second portion that is wider than the first portion; the second portions of the first feed element and the second feed element have widths that are greater than widths of the first passive element and the second passive element in a direction perpendicular to a direction in which the folded-back parts (121, 122, 123, 124, 125) extend, respectively; and at least the second portions of the first feed element and the second feed element are configured to couple to the folded-back parts (121, 122, 123, 124, 125) of the first passive element and the second passive element, respectively.