A driving assistance device comprising: a blind spot recognition unit that recognizes a blind spot of a driver in a progressing direction of a host vehicle; and a mobile object information setting unit that sets mobile object information including at least an assumed speed of a mobile object, as information on the mobile object having a possibility of appearing suddenly from the blind spot, characterized by a speed zone computation unit that computes a speed zone within a map which is determined by a relationship between a speed of the host vehicle, and a distance of the host vehicle with respect to a reference position in a place forming the blind spot, in which there is a possibility that the host vehicle will come into contact with the mobile object when progressing in the progressing direction, based on the mobile object information set by the mobile object information setting unit; a target speed computation unit that computes a target speed of the host vehicle based on the speed zone; a driving assistance starting point computation unit that computes a driving assistance starting point within said map which is determined by a relationship between a speed of the host vehicle, and a distance of the host vehicle with respect to a reference position in a place forming the blind spot, wherein the driving assistance starting point is determined by the speed of the host vehicle at the time when the driving assistance is started, and the distance of the host vehicle with respect to the reference position at the time when the driving assistance is started; and a target speed modification unit that modifies the target speed, when the driving assistance starting point is within the speed zone, wherein the target speed computation unit sets a speed lower than the speed zone in the reference position as the target speed, and wherein the target speed modification unit sets a value higher than a value computed by the target speed computation unit as a new target speed, in speed in the reference position. a blind spot recognition unit that recognizes a blind spot of a driver in a progressing direction of a host vehicle; and a mobile object information setting unit that sets mobile object information including at least an assumed speed of a mobile object, as information on the mobile object having a possibility of appearing suddenly from the blind spot, characterized by a speed zone computation unit that computes a speed zone within a map which is determined by a relationship between a speed of the host vehicle, and a distance of the host vehicle with respect to a reference position in a place forming the blind spot, in which there is a possibility that the host vehicle will come into contact with the mobile object when progressing in the progressing direction, based on the mobile object information set by the mobile object information setting unit; a target speed computation unit that computes a target speed of the host vehicle based on the speed zone; a driving assistance starting point computation unit that computes a driving assistance starting point within said map which is determined by a relationship between a speed of the host vehicle, and a distance of the host vehicle with respect to a reference position in a place forming the blind spot, wherein the driving assistance starting point is determined by the speed of the host vehicle at the time when the driving assistance is started, and the distance of the host vehicle with respect to the reference position at the time when the driving assistance is started; and a target speed modification unit that modifies the target speed, when the driving assistance starting point is within the speed zone, wherein the target speed computation unit sets a speed lower than the speed zone in the reference position as the target speed, and wherein the target speed modification unit sets a value higher than a value computed by the target speed computation unit as a new target speed, in speed in the reference position.