A floating bush bearing device, comprising: a casing including a bearing hole; a rotary shaft disposed rotatably inside the bearing hole; a floating bush (46,120,130,140) disposed rotatably inside the bearing hole and surrounding the rotary shaft; an oil-feed hole of lubricant oil, the oil-feed hole having an opening on an inner peripheral surface of the bearing hole; a plurality of communication holes formed on the floating bush (46, 120,130,140) each of the communication holes extending between an inner peripheral surface and an outer peripheral surface of the floating bush (46, 120, 130, 140), and disposed at intervals in a circumferential direction of the floating bush (46, 120,130,140); and a circumferential groove (110,122,132,142) formed on the outer peripheral surface of the floating bush (46,120,130,140) and extending over an entire circumference of the outer peripheral surface of the floating bush (46,120,130,140), the circumferential groove (110,122,132,142) passing through openings of the plurality of communication holes wherein the circumferential groove (110,132,142) has a cross-sectional area which varies in accordance with a circumferential position, wherein the circumferential groove (110,122,132,142) includes a plurality of first regions (110a,122a,132c,142a) in which the circumferential groove (110,122,132,142) positionally overlaps with the respective openings of the plurality of communication holes in the circumferential direction of the floating bush (46,120,130,140) and a plurality of second regions (110b,142b) each extending between the plurality of first regions (110a,122a,132c,142a), wherein a cross-sectional area of each of the plurality of first regions (110a,122a,132c,142a) is greater than a cross-sectional area of each of the plurality of second regions (110b,142b), characterised in that the width of each of the plurality of first regions (110a,122a,132c,142a) in an axial direction of the floating bush (46,120,130,140) is in a range of from 0.9 to 1.3 times a width of each of the plurality of communication holes, wherein at least one of the plurality of first regions (110a,132c,142a) includes a part at which a width Wa of the circumferential groove (110,132,142) in the axial direction reaches a maximum width Wmax, and wherein the width Wb of each of the plurality of second regions (110b,142b) in the axial direction is in a range of from 0.2 to 0.4 times the maximum width Wmax of the circumferential groove (110,132,142). a casing including a bearing hole; a rotary shaft disposed rotatably inside the bearing hole; a floating bush (46,120,130,140) disposed rotatably inside the bearing hole and surrounding the rotary shaft; an oil-feed hole of lubricant oil, the oil-feed hole having an opening on an inner peripheral surface of the bearing hole; a plurality of communication holes formed on the floating bush (46, 120,130,140) each of the communication holes extending between an inner peripheral surface and an outer peripheral surface of the floating bush (46, 120, 130, 140), and disposed at intervals in a circumferential direction of the floating bush (46, 120,130,140); and a circumferential groove (110,122,132,142) formed on the outer peripheral surface of the floating bush (46,120,130,140) and extending over an entire circumference of the outer peripheral surface of the floating bush (46,120,130,140), the circumferential groove (110,122,132,142) passing through openings of the plurality of communication holes wherein the circumferential groove (110,132,142) has a cross-sectional area which varies in accordance with a circumferential position, wherein the circumferential groove (110,122,132,142) includes a plurality of first regions (110a,122a,132c,142a) in which the circumferential groove (110,122,132,142) positionally overlaps with the respective openings of the plurality of communication holes in the circumferential direction of the floating bush (46,120,130,140) and a plurality of second regions (110b,142b) each extending between the plurality of first regions (110a,122a,132c,142a), wherein a cross-sectional area of each of the plurality of first regions (110a,122a,132c,142a) is greater than a cross-sectional area of each of the plurality of second regions (110b,142b), characterised in that the width of each of the plurality of first regions (110a,122a,132c,142a) in an axial direction of the floating bush (46,120,130,140) is in a range of from 0.9 to 1.3 times a width of each of the plurality of communication holes, wherein at least one of the plurality of first regions (110a,132c,142a) includes a part at which a width Wa of the circumferential groove (110,132,142) in the axial direction reaches a maximum width Wmax, and wherein the width Wb of each of the plurality of second regions (110b,142b) in the axial direction is in a range of from 0.2 to 0.4 times the maximum width Wmax of the circumferential groove (110,132,142).