A horizontally sliding window, comprising: a square frame-shaped window casing formed by assembling transverse frames, which are divided into a first rail part located on an indoor side and a second rail part located on an outdoor side, with longitudinal frames; a first window disposed on the first rail part on an inner side of the window casing; a second window disposed on the second rail part on an inner side of the window casing; a first movable frame which is interposed between an upper end of the first rail part and an upper end of the first window and between a lower end of the first rail part and a lower end of the first window, from both sides of which a first cover member for covering an indoor side of the first rail part and a second cover member to be coupled with the second rail part respectively extend, and which includes rollers rotatably coupled between the first and second cover members (241, 242) so as to operate in contact with the first rail part; and a second movable frame which is interposed between an upper end of the second rail part and an upper end of the second window and between a lower end of the second rail part and a lower end of the second window, from both sides of which a third cover member for covering an indoor side of the second rail part and a fourth cover member for covering an outdoor side of the second rail part respectively extend, and which includes rollers rotatably coupled between the third and fourth cover members (251, 252) so as to operate in contact with the second rail part, wherein a height of the second rail part of the transverse frames is higher than that of the first rail part thereof, wherein a hook piece protrudes in an indoor side direction of the second rail part to be bent in the vicinity of the first rail part, and the second cover member extends straight and then is bent toward an outdoor side so as to engage with the hook piece, wherein a guide protrusion for guiding the first cover member when the first movable frame moves in a sliding manner protrudes from an indoor side of the first rail part,wherein the first cover member for covering an indoor side of the first rail part extends from one side of the first movable frame, and the second cover member to engage with the hook piece of the second rail part extends from another side of the first movable frame, and wherein the first cover member extends straight in the vicinity of the guide protrusion of the first rail part. a square frame-shaped window casing formed by assembling transverse frames, which are divided into a first rail part located on an indoor side and a second rail part located on an outdoor side, with longitudinal frames; a first window disposed on the first rail part on an inner side of the window casing; a second window disposed on the second rail part on an inner side of the window casing; a first movable frame which is interposed between an upper end of the first rail part and an upper end of the first window and between a lower end of the first rail part and a lower end of the first window, from both sides of which a first cover member for covering an indoor side of the first rail part and a second cover member to be coupled with the second rail part respectively extend, and which includes rollers rotatably coupled between the first and second cover members (241, 242) so as to operate in contact with the first rail part; and a second movable frame which is interposed between an upper end of the second rail part and an upper end of the second window and between a lower end of the second rail part and a lower end of the second window, from both sides of which a third cover member for covering an indoor side of the second rail part and a fourth cover member for covering an outdoor side of the second rail part respectively extend, and which includes rollers rotatably coupled between the third and fourth cover members (251, 252) so as to operate in contact with the second rail part, wherein a height of the second rail part of the transverse frames is higher than that of the first rail part thereof, wherein a hook piece protrudes in an indoor side direction of the second rail part to be bent in the vicinity of the first rail part, and the second cover member extends straight and then is bent toward an outdoor side so as to engage with the hook piece, wherein a guide protrusion for guiding the first cover member when the first movable frame moves in a sliding manner protrudes from an indoor side of the first rail part,wherein the first cover member for covering an indoor side of the first rail part extends from one side of the first movable frame, and the second cover member to engage with the hook piece of the second rail part extends from another side of the first movable frame, and wherein the first cover member extends straight in the vicinity of the guide protrusion of the first rail part.