A controller configured to control vehicle body behavior of a motorcycle, the controller comprising: an acquisition section configured to acquire trigger information generated in accordance with peripheral environment of the motorcycle; and an execution section configured to initiate a control mode making the motorcycle execute an automatic brake operation in accordance with the trigger information acquired by the acquisition section and to make the motorcycle generate a braking force, wherein the acquisition section is further configured to acquire seat load information that is information of a load received by a seat (1a, 1b) of the motorcycle, and the execution section is configured to change the automatic brake operation, which is executed in the control mode, in accordance with the seat load information acquired by the acquisition section, wherein the acquisition section is configured to acquire the seat load information on the basis of a detection result of an external force detector configured to detect an external force acting on the seat (1a, 1b) of the motorcycle, characterized in that the external force detector includes a sensor which is configured to detect the load received by a tandem seat of the motorcycle and a sensor configured to detect the load received by the driver's seat of the motorcycle, wherein the execution section is configured to change the automatic brake operation, which is executed in the control mode, in accordance with a result of a comparison between a value that is acquired as the seat load information and a threshold value, wherein a seat load determination section of the execution section is configured to determine a case where a sum of the external forces is equal to or smaller than the threshold value as the state where an occupants' weight is light, and to determine a case where the sum of the external forces is larger than the threshold value as the state where the occupants' weight is heavy. an acquisition section configured to acquire trigger information generated in accordance with peripheral environment of the motorcycle; and an execution section configured to initiate a control mode making the motorcycle execute an automatic brake operation in accordance with the trigger information acquired by the acquisition section and to make the motorcycle generate a braking force, wherein the acquisition section is further configured to acquire seat load information that is information of a load received by a seat (1a, 1b) of the motorcycle, and the execution section is configured to change the automatic brake operation, which is executed in the control mode, in accordance with the seat load information acquired by the acquisition section, the acquisition section is further configured to acquire seat load information that is information of a load received by a seat (1a, 1b) of the motorcycle, and the execution section is configured to change the automatic brake operation, which is executed in the control mode, in accordance with the seat load information acquired by the acquisition section, wherein the acquisition section is configured to acquire the seat load information on the basis of a detection result of an external force detector configured to detect an external force acting on the seat (1a, 1b) of the motorcycle, characterized in that the external force detector includes a sensor which is configured to detect the load received by a tandem seat of the motorcycle and a sensor configured to detect the load received by the driver's seat of the motorcycle, the acquisition section is configured to acquire the seat load information on the basis of a detection result of an external force detector configured to detect an external force acting on the seat (1a, 1b) of the motorcycle, characterized in that the external force detector includes a sensor which is configured to detect the load received by a tandem seat of the motorcycle and a sensor configured to detect the load received by the driver's seat of the motorcycle, wherein the execution section is configured to change the automatic brake operation, which is executed in the control mode, in accordance with a result of a comparison between a value that is acquired as the seat load information and a threshold value, wherein a seat load determination section of the execution section is configured to determine a case where a sum of the external forces is equal to or smaller than the threshold value as the state where an occupants' weight is light, and to determine a case where the sum of the external forces is larger than the threshold value as the state where the occupants' weight is heavy.