A side door structure of a vehicle, comprising: an outer member; an inner member connected to the outer member and including a front-side vertical wall portion and a rear-side vertical wall portion which are positioned respectively at a front side and a rear side, in a vehicle longitudinal direction, of the inner member, the front-side vertical wall portion being provided with a hinge rotatable relative to a vehicle body, the rear-side vertical wall portion being provided with a latch device capable of lock/unlock relative to the vehicle body; an impact bar provided to extend from a front-side attachment portion (41s, 41t) which is formed at the front-side vertical wall portion to a rear-side attachment portion (42s, 42t) which is formed at the rear-side vertical wall portion; a hinge-reinforcement member connected to the hinge and reinforcing a hinge attachment portion where the hinge is attached to the front-side vertical wall portion; and a latch-reinforcement member connected to the latch device and reinforcing a latch-attachment portion where the latch device is attached to the rear-side vertical wall portion, wherein at least one of the front-side vertical wall portion and the rear-side vertical wall portion is configured as a cast or resin-made vertical wall portion which is made by casting or of resin, and the hinge-reinforcement member and/or the latch-reinforcement member which are provided at the respective cast or resin-made vertical wall portion are configured to have higher ductility than the cast or resin -made vertical wall portion and connected to the impact bar, and characterised in that the front-side vertical wall portion includes a plurality of the front-side attachment portions (41s, 41t) which are arranged along a longitudinal direction of the impact bar and a single or plurality of rib portions which extend in a vehicle width direction and connect the plural front-side attachment portions (41s, 41t). an outer member; an inner member connected to the outer member and including a front-side vertical wall portion and a rear-side vertical wall portion which are positioned respectively at a front side and a rear side, in a vehicle longitudinal direction, of the inner member, the front-side vertical wall portion being provided with a hinge rotatable relative to a vehicle body, the rear-side vertical wall portion being provided with a latch device capable of lock/unlock relative to the vehicle body; an impact bar provided to extend from a front-side attachment portion (41s, 41t) which is formed at the front-side vertical wall portion to a rear-side attachment portion (42s, 42t) which is formed at the rear-side vertical wall portion; a hinge-reinforcement member connected to the hinge and reinforcing a hinge attachment portion where the hinge is attached to the front-side vertical wall portion; and a latch-reinforcement member connected to the latch device and reinforcing a latch-attachment portion where the latch device is attached to the rear-side vertical wall portion, wherein at least one of the front-side vertical wall portion and the rear-side vertical wall portion is configured as a cast or resin-made vertical wall portion which is made by casting or of resin, and the hinge-reinforcement member and/or the latch-reinforcement member which are provided at the respective cast or resin-made vertical wall portion are configured to have higher ductility than the cast or resin -made vertical wall portion and connected to the impact bar, and characterised in that the front-side vertical wall portion includes a plurality of the front-side attachment portions (41s, 41t) which are arranged along a longitudinal direction of the impact bar and a single or plurality of rib portions which extend in a vehicle width direction and connect the plural front-side attachment portions (41s, 41t).