A vehicle body behavior control system for a motorcycle, the vehicle body behavior control system comprising: a peripheral environment detector that detects peripheral environment of the motorcycle; and a controller that controls vehicle body behavior of the motorcycle, the vehicle body behavior control system further comprising: an external force detector that detects an external force acting on the motorcycle, wherein the controller includes: an acquisition section that acquires trigger information generated in accordance with a detection result of the peripheral environment detector; and an execution section that initiates a control mode making the motorcycle execute an automatic brake operation in accordance with the trigger information acquired by the acquisition section and makes the motorcycle generate a braking force, the acquisition section further acquires deceleration resistance information that is information related to a driver's driving posture on the motorcycle on the basis of a detection result of the external force detector, and the execution section changes a control command that is output in the control mode in accordance with the deceleration resistance information acquired by the acquisition section, wherein the external force detector includes a sensor (42b, 42c) that detects presence/absence, a magnitude, or an action position of the external force acting on a portion of the motorcycle abutting the driver's knee and additionally or alternatively presence/absence, a magnitude, or an action position of the external force acting on a seat (la) of the motorcycle. a peripheral environment detector that detects peripheral environment of the motorcycle; and a controller that controls vehicle body behavior of the motorcycle, the vehicle body behavior control system further comprising: an external force detector that detects an external force acting on the motorcycle, wherein the controller includes: an acquisition section that acquires trigger information generated in accordance with a detection result of the peripheral environment detector; and an execution section that initiates a control mode making the motorcycle execute an automatic brake operation in accordance with the trigger information acquired by the acquisition section and makes the motorcycle generate a braking force, the acquisition section further acquires deceleration resistance information that is information related to a driver's driving posture on the motorcycle on the basis of a detection result of the external force detector, and the execution section changes a control command that is output in the control mode in accordance with the deceleration resistance information acquired by the acquisition section, an external force detector that detects an external force acting on the motorcycle, wherein the controller includes: an acquisition section that acquires trigger information generated in accordance with a detection result of the peripheral environment detector; and an execution section that initiates a control mode making the motorcycle execute an automatic brake operation in accordance with the trigger information acquired by the acquisition section and makes the motorcycle generate a braking force, an acquisition section that acquires trigger information generated in accordance with a detection result of the peripheral environment detector; and an execution section that initiates a control mode making the motorcycle execute an automatic brake operation in accordance with the trigger information acquired by the acquisition section and makes the motorcycle generate a braking force, the acquisition section further acquires deceleration resistance information that is information related to a driver's driving posture on the motorcycle on the basis of a detection result of the external force detector, and the execution section changes a control command that is output in the control mode in accordance with the deceleration resistance information acquired by the acquisition section, wherein the external force detector includes a sensor (42b, 42c) that detects presence/absence, a magnitude, or an action position of the external force acting on a portion of the motorcycle abutting the driver's knee and additionally or alternatively presence/absence, a magnitude, or an action position of the external force acting on a seat (la) of the motorcycle.