A retention device to be attached to a structure member included in a work machine for conducting lifting work of lifting up the structure member by lifting equipment, the retention device comprising: a proximal end portion; a first leg portion that is supported by the proximal end portion and includes a first leg portion main body that is disposed below the proximal end portion; a second leg portion that is supported by the proximal end portion and includes a second leg portion main body disposed below the proximal end portion, the second leg portion main body being disposed at an interval in a horizontal direction from the first leg portion main body; and a support member supported by the first leg portion main body so as to be movable between an entry allowed position and a retention position, the entry allowed position being a position where the support member retracts from the second leg portion main body to allow a retained portion as a part of the structure member to enter between the first leg portion main body and the second leg portion main body, and the retention position being a position where the retained portion is retained on the support member while the support member is supported by the first leg portion main body and the second leg portion main body; characterized in that the retention device further comprises an upper pressing member that is interposed between the proximal end portion and the support member so as to be supported by at least one of the proximal end portion, the first leg portion, and the second leg portion, the upper pressing member having an opposing surface opposed to an upper portion of the retained portion that is retained on the support member. a proximal end portion; a first leg portion that is supported by the proximal end portion and includes a first leg portion main body that is disposed below the proximal end portion; a second leg portion that is supported by the proximal end portion and includes a second leg portion main body disposed below the proximal end portion, the second leg portion main body being disposed at an interval in a horizontal direction from the first leg portion main body; and a support member supported by the first leg portion main body so as to be movable between an entry allowed position and a retention position, the entry allowed position being a position where the support member retracts from the second leg portion main body to allow a retained portion as a part of the structure member to enter between the first leg portion main body and the second leg portion main body, and the retention position being a position where the retained portion is retained on the support member while the support member is supported by the first leg portion main body and the second leg portion main body; characterized in that the retention device further comprises an upper pressing member that is interposed between the proximal end portion and the support member so as to be supported by at least one of the proximal end portion, the first leg portion, and the second leg portion, the upper pressing member having an opposing surface opposed to an upper portion of the retained portion that is retained on the support member.