Controller and control method for behavior of straddle-type vehicle

A control method for behavior of a straddle-type vehicle (100), the control method comprising:

• an acquisition step (S101) in which a controller (20) acquires positional relationship information between the traveling straddle-type vehicle (100) and a target (200);
• and
• an execution step (S102) in which the controller (20) executes positional relationship adjustment operation to automatically change a travel speed of the straddle-type vehicle (100) on the basis of the positional relationship information acquired in the acquisition step (S101) and to thereby adjust a positional relationship between the straddle-type vehicle (100) and the target (200), wherein
• in the execution step (S102), in the positional relationship adjustment operation, the controller switches between a first operation mode in which the positional relationship between the straddle-type vehicle (100) and the target (200) is brought into a first state and a second operation mode in which the positional relationship between the straddle-type vehicle (100) and the target (200) is brought into a second state differing from the first state according to whether a group travel mode is valid, the group travel mode being a mode in which the straddle-type vehicle (100) travels with at least one other straddle-type vehicle (300) in a group, wherein in the positional relationship adjustment operation, the controller prohibits switchover between the first operation mode and the second operation mode according to road line shape information.