A work robot designed to perform a given work while traveling autonomously over a given area, the work robot comprising: a traveling device configured to switch between forward and backward movements of the work robot, and change an orientation of the work robot; a collision sensor (5, 6) configured to output a signal regarding a collision between the work robot and an obstacle; and a control device configured to determine, based on the signal received from the collision sensor (5, 6), whether or not the work robot has collided with an obstacle, and further determine a collision side indicative of whether the work robot has collided with the obstacle on a left side with respect to a center of the work robot, or has collided with the obstacle on a right side with respect to the center of the work robot, so as to control the traveling device, wherein the control device is operable, when it determines that the work robot has collided with the obstacle, to control the traveling device to change the orientation of the work robot toward a side opposite to the collision side, and then move the work robot forwardly, characterized in that the collision sensor (5, 6) comprises a magnet (52, 62), and a hall sensor (51, 61) having a detection part (51 a, 61a) to detect an intensity of a magnetic field generated by the magnet (52, 62), the hall sensor (51, 61) being configured to output a signal corresponding to the intensity of the magnetic field, in a direction parallel to a straight line (M1, M2) passing through an N pole and an S pole of the magnet (52, 62), the detection part (51a, 61a) of the hall sensor (51, 61) is disposed at a position between an N pole-side end (52n, 62n) and an S pole-side end (52s, 62s) of the magnet (52, 62), and the position of the detection part (51a, 61a) of the hall sensor (51, 61) with respect to the magnet (52, 62) is changeable in the direction parallel to the straight line (M1, M2), based on the collision between the work robot and the obstacle, and that the collision sensor (5, 6) is provided by a number of two, the magnet of a first collision sensor which is one of the two collision sensors (5, 6) is disposed such that the straight line passing through the N pole end and the S pole thereof passes through the center of the work robot and becomes approximately parallel to a center line extending along a forward/backward direction of the work robot, and the magnet of a second collision sensor which is a remaining one of the two collision sensors (5, 6) is disposed such that the straight line passing through the N pole end and the S pole thereof becomes approximately orthogonal to the center line. a traveling device configured to switch between forward and backward movements of the work robot, and change an orientation of the work robot; a collision sensor (5, 6) configured to output a signal regarding a collision between the work robot and an obstacle; and a control device configured to determine, based on the signal received from the collision sensor (5, 6), whether or not the work robot has collided with an obstacle, and further determine a collision side indicative of whether the work robot has collided with the obstacle on a left side with respect to a center of the work robot, or has collided with the obstacle on a right side with respect to the center of the work robot, so as to control the traveling device, wherein the control device is operable, when it determines that the work robot has collided with the obstacle, to control the traveling device to change the orientation of the work robot toward a side opposite to the collision side, and then move the work robot forwardly, characterized in that the collision sensor (5, 6) comprises a magnet (52, 62), and a hall sensor (51, 61) having a detection part (51 a, 61a) to detect an intensity of a magnetic field generated by the magnet (52, 62), the hall sensor (51, 61) being configured to output a signal corresponding to the intensity of the magnetic field, in a direction parallel to a straight line (M1, M2) passing through an N pole and an S pole of the magnet (52, 62), the detection part (51a, 61a) of the hall sensor (51, 61) is disposed at a position between an N pole-side end (52n, 62n) and an S pole-side end (52s, 62s) of the magnet (52, 62), and the position of the detection part (51a, 61a) of the hall sensor (51, 61) with respect to the magnet (52, 62) is changeable in the direction parallel to the straight line (M1, M2), based on the collision between the work robot and the obstacle, and that the collision sensor (5, 6) is provided by a number of two, the magnet of a first collision sensor which is one of the two collision sensors (5, 6) is disposed such that the straight line passing through the N pole end and the S pole thereof passes through the center of the work robot and becomes approximately parallel to a center line extending along a forward/backward direction of the work robot, and the magnet of a second collision sensor which is a remaining one of the two collision sensors (5, 6) is disposed such that the straight line passing through the N pole end and the S pole thereof becomes approximately orthogonal to the center line.