A collision avoidance assistance device includes a target control amount calculation unit configured to calculate a target control amount used for travelling on a target trajectory through which the vehicle passes, a first control amount calculation unit configured to calculate a first control amount to be applied by the first steering assistance system out of the target control amount, and output the first control amount to the first steering assistance system, and a second control amount calculation unit configured to calculate a second control amount to be applied by the second steering assistance system out of the target control amount, and output the second control amount to the second steering assistance system. The first control amount is set to be larger than the second control amount during a predetermined time period from a time of starting the collision avoidance assistance.