Abstract: A door roller provided with an engagement arm which is capable of being engaged with, by an engaging member attached to an upwardly bent end of the engaging arm, a guide groove formed with a downward opening longitudinally along a lower frame of a window structure for preventing a sliding door from an accidental removal. An inner housing retaining a tire shaft is rotatably pivoted to an outer housing of the door roller via another axis parallel to the tire shaft. The engagement arm which is engaged with the inner housing by way of an engaging mechanism is disposed freely movable upwards and downwards in relation to the outer housing. The engaging mechanism is located at a further position than the tire shaft from the another axis such that the engagement arm may be moved vertically, when the tire has been ascended or descended by a predetermined amount, by a larger amount than or an approx. equal amount to the predetermined amount in relation to the outer housing.