A catheter assembly (10A to 10C) comprising: a catheter; a catheter hub fixed to a proximal end portion of the catheter; a hollow inner needle having a needle tip and disengageably inserted into the catheter; a needle hub fixed to a proximal end portion of the inner needle; a guide wire slidably inserted into the inner needle, being longer than the catheter, and having a distal end protrudable from the needle tip; a guide wire hub configured to support the guide wire and move the guide wire with respect to the inner needle in association with movement; and a movement mechanism (92A to 92C) configured to retract the guide wire hub with respect to the needle hub such that the distal end of the guide wire is housed in the inner needle in association with forward movement of the catheter hub with respect to the needle hub, wherein the movement mechanism (92A to 92C) includes a force transmitter (26, 94,112,132,134) configured to transmit force to the guide wire hub in association with movement of the catheter hub, and a force direction changer (96, 136) configured to change a direction of the force of the force transmitter (26, 94, 112, 132, 134), characterized in that the movement mechanism (92A, 92C) has a movable member configured to move forward with respect to the needle hub in association with the forward movement of the catheter hub with respect to the needle hub, and when the catheter hub moves forward with respect to the needle hub, the movement mechanism converts a force moving the movable member forward with respect to the needle hub into a force retracting the guide wire hub with respect to the needle hub. a catheter; a catheter hub fixed to a proximal end portion of the catheter; a hollow inner needle having a needle tip and disengageably inserted into the catheter; a needle hub fixed to a proximal end portion of the inner needle; a guide wire slidably inserted into the inner needle, being longer than the catheter, and having a distal end protrudable from the needle tip; a guide wire hub configured to support the guide wire and move the guide wire with respect to the inner needle in association with movement; and a movement mechanism (92A to 92C) configured to retract the guide wire hub with respect to the needle hub such that the distal end of the guide wire is housed in the inner needle in association with forward movement of the catheter hub with respect to the needle hub, wherein the movement mechanism (92A to 92C) includes a force transmitter (26, 94,112,132,134) configured to transmit force to the guide wire hub in association with movement of the catheter hub, and a force direction changer (96, 136) configured to change a direction of the force of the force transmitter (26, 94, 112, 132, 134), characterized in that the movement mechanism (92A, 92C) has a movable member configured to move forward with respect to the needle hub in association with the forward movement of the catheter hub with respect to the needle hub, and when the catheter hub moves forward with respect to the needle hub, the movement mechanism converts a force moving the movable member forward with respect to the needle hub into a force retracting the guide wire hub with respect to the needle hub.