A window shade comprising: a head frame; a reel pivotally connected with the head frame and rotatable about a first pivot axis; a roller disposed at a position spaced apart from the reel and pivotally connected with the head frame and rotatable about a second pivot axis; a panel assembly connected with the reel and including a first and a second panel (116, 118), and a plurality of transversal vanes that are respectively connected with the first and second panels (116, 118), the reel being rotatable to wind and unwind the panel assembly with the first panel remaining in contact with the roller, and the panel assembly being switchable between an open state for light passage and a closed state blocking light passage by rotating the transversal vanes; and a clutch operatively connected with the roller, the clutch being switchable between an engaged state preventing the roller from rotating in a first direction, and a disengaged state for rotation of the roller in a second direction opposite to the first direction, wherein the reel is rotatable in an unwinding direction while the roller is kept stationary by the clutch in the engaged state for switching the panel assembly from the closed state to the open state; wherein the roller is pivotally assembled about a fixed shaft portion attached to the head frame, characterized in that the clutch includes: a guide track provided on the fixed shaft portion, the guide track including a closed-loop portion running circumferentially around the fixed shaft portion, and a stop portion connected with the closed-loop portion; a guide slot provided inside the roller and overlapping partially with the guide track; and a running member guided for movement along the guide slot and the guide track, wherein the running member is engaged with the stop portion when the clutch is in the engaged state. a head frame; a reel pivotally connected with the head frame and rotatable about a first pivot axis; a roller disposed at a position spaced apart from the reel and pivotally connected with the head frame and rotatable about a second pivot axis; a panel assembly connected with the reel and including a first and a second panel (116, 118), and a plurality of transversal vanes that are respectively connected with the first and second panels (116, 118), the reel being rotatable to wind and unwind the panel assembly with the first panel remaining in contact with the roller, and the panel assembly being switchable between an open state for light passage and a closed state blocking light passage by rotating the transversal vanes; and a clutch operatively connected with the roller, the clutch being switchable between an engaged state preventing the roller from rotating in a first direction, and a disengaged state for rotation of the roller in a second direction opposite to the first direction, wherein the reel is rotatable in an unwinding direction while the roller is kept stationary by the clutch in the engaged state for switching the panel assembly from the closed state to the open state; wherein the roller is pivotally assembled about a fixed shaft portion attached to the head frame, characterized in that the clutch includes: a guide track provided on the fixed shaft portion, the guide track including a closed-loop portion running circumferentially around the fixed shaft portion, and a stop portion connected with the closed-loop portion; a guide slot provided inside the roller and overlapping partially with the guide track; and a running member guided for movement along the guide slot and the guide track, wherein the running member is engaged with the stop portion when the clutch is in the engaged state. a guide track provided on the fixed shaft portion, the guide track including a closed-loop portion running circumferentially around the fixed shaft portion, and a stop portion connected with the closed-loop portion; a guide slot provided inside the roller and overlapping partially with the guide track; and a running member guided for movement along the guide slot and the guide track, wherein the running member is engaged with the stop portion when the clutch is in the engaged state.