A wheel assembly of a mobile robot, comprising: a gear box which accommodates a driving gear driven by a driving motor and a driven gear driven by the driving gear, is installed to be rotatable about a rotation shaft, which is provided at one side of the gear box, in front and rear directions with respect to a robot body, and has a first fixing part, wherein at least a part of an outer circumferential surface of the rotation shaft is circular, and the rotation shaft is seated and supported on a rotation shaft seating portion having a shape that matches a shape of the outer circumferential surface of the rotation shaft; a wheel which is coupled to a rotary shaft of the driven gear so as to be rotated integrally with the driven gear, and moved up or down with respect to the robot body when the gear box is rotated; a pulling member which has one end fixed to the first fixing part, and the other end fixed to a second fixing part a position of which is fixed with respect to the robot body, and pulls the first fixing part toward the second fixing part side so as to rotate the gear box in a direction in which the wheel is moved down with respect to the robot body; characterized by a lower case which accommodates at least a part of the gear box, and at least a part of the wheel, has an opened upper portion, and an opened portion of a lower surface where a route through which the wheel is moved up and down exists, and is coupled and fixed to the robot body, wherein the rotation shaft seating portion is provided at the lower case, wherein the second fixing part is provided at the lower case; and an upper case which is coupled to an upper portion of the lower case, and forms an accommodating space which accommodates at least a part of the gear box and at least a part of the wheel, wherein the upper case includes a rotation shaft cover which is provided at an opposite side to the rotation shaft seating portion with the rotation shaft interposed therebetween so as to prevent the rotation shaft from being moved away from the rotation shaft seating portion. a gear box which accommodates a driving gear driven by a driving motor and a driven gear driven by the driving gear, is installed to be rotatable about a rotation shaft, which is provided at one side of the gear box, in front and rear directions with respect to a robot body, and has a first fixing part, wherein at least a part of an outer circumferential surface of the rotation shaft is circular, and the rotation shaft is seated and supported on a rotation shaft seating portion having a shape that matches a shape of the outer circumferential surface of the rotation shaft; a wheel which is coupled to a rotary shaft of the driven gear so as to be rotated integrally with the driven gear, and moved up or down with respect to the robot body when the gear box is rotated; a pulling member which has one end fixed to the first fixing part, and the other end fixed to a second fixing part a position of which is fixed with respect to the robot body, and pulls the first fixing part toward the second fixing part side so as to rotate the gear box in a direction in which the wheel is moved down with respect to the robot body; characterized by a lower case which accommodates at least a part of the gear box, and at least a part of the wheel, has an opened upper portion, and an opened portion of a lower surface where a route through which the wheel is moved up and down exists, and is coupled and fixed to the robot body, wherein the rotation shaft seating portion is provided at the lower case, wherein the second fixing part is provided at the lower case; and an upper case which is coupled to an upper portion of the lower case, and forms an accommodating space which accommodates at least a part of the gear box and at least a part of the wheel, wherein the upper case includes a rotation shaft cover which is provided at an opposite side to the rotation shaft seating portion with the rotation shaft interposed therebetween so as to prevent the rotation shaft from being moved away from the rotation shaft seating portion.