1. A vehicle display device, comprising: a base capable of being coupled to a vehicle, a reflective panel being movable between an extended position in which the reflective panel extends from the base and a retracted position in which the reflective panel is retracted into the base, a projecting means configured to project an image to be displayed on the reflective panel, a cover configured to cover a front side of the reflective panel in a direction toward the extended position when the reflective panel is in the retracted position, and having an arm portion extending to the base and pivotally supported by the base, and a first biasing means for biasing the cover, and a driving means is disposed in the base and configured to drive the reflective panel in an extendable-and-retractable manner, wherein the cover is movable between a closed position in which it can cover the front side of the reflective panel in the direction toward the extended position and an open position in which it deviates from a movement pathway of the reflective panel and a projection pathway of the projecting means toward the reflective panel, wherein due to the first biasing means _the cover is biased toward the open position, wherein the base _has an interlocking member being in association with movement of the reflective panel in the extendable-and-retractable manner and configured to press the arm portion, wherein the interlocking member is moved in one direction when the reflective panel is moved to the retracted position, wherein when the arm portion is pressed by the interlocking member, the cover is moved to the closed position against a force exerted by the first biasing means, wherein the interlocking member is movable in the other direction when the reflective panel is moved to the extended position, and wherein due to the force exerted by the first biasing means the interlocking member is moved via the arm portion in the other direction and the cover is moved to the open position, characterized in that the interlocking member is provided with a second biasing member configured to bias the interlocking member toward the other direction. a base capable of being coupled to a vehicle, a reflective panel being movable between an extended position in which the reflective panel extends from the base and a retracted position in which the reflective panel is retracted into the base, a projecting means configured to project an image to be displayed on the reflective panel, a cover configured to cover a front side of the reflective panel in a direction toward the extended position when the reflective panel is in the retracted position, and having an arm portion extending to the base and pivotally supported by the base, and a first biasing means for biasing the cover, and a driving means is disposed in the base and configured to drive the reflective panel in an extendable-and-retractable manner, wherein the cover is movable between a closed position in which it can cover the front side of the reflective panel in the direction toward the extended position and an open position in which it deviates from a movement pathway of the reflective panel and a projection pathway of the projecting means toward the reflective panel, wherein due to the first biasing means _the cover is biased toward the open position, wherein the base _has an interlocking member being in association with movement of the reflective panel in the extendable-and-retractable manner and configured to press the arm portion, wherein the interlocking member is moved in one direction when the reflective panel is moved to the retracted position, wherein when the arm portion is pressed by the interlocking member, the cover is moved to the closed position against a force exerted by the first biasing means, wherein the interlocking member is movable in the other direction when the reflective panel is moved to the extended position, and wherein due to the force exerted by the first biasing means the interlocking member is moved via the arm portion in the other direction and the cover is moved to the open position, characterized in that the interlocking member is provided with a second biasing member configured to bias the interlocking member toward the other direction.