To further reduce the bearing on top of a boss hole of a pin boss part and to reduce the weight of a connecting rod in the connection structure of a piston and the connecting rod in an internal combustion engine.

In the connection structure of the piston and the connecting rod, a small end part of the connecting rod is disposed between pin boss parts of the piston, and a piston pin is inserted into a boss hole formed in the pin boss parts and an attaching hole formed piercing through both end faces of the small end part to connect the piston and the connecting rod. A step part is formed on each end face of the small end part of the connecting rod, and the width between both end faces on the upper side than the step part in the small end part is smaller than the width between end faces on the root side. The both end faces on the upper side than the step part in the small end part and both end faces on the root side are flat planes orthogonally crossing the longitudinal axial direction of the attaching hole.