To provide a combustion chamber structure of a bi-fuel engine for improving output, and lengthening a cruising distance of a mounting vehicle by allowing not only stratified combustion by gas fuel but also stratified combustion by liquid fuel.

A spark plug 109 is arranged in the upper center of a combustion chamber 105 between an intake port 106 and an exhaust port 107. A CNG injection valve 120 and a gasoline injection valve 130 are arranged in parallel in a mode of sandwiching the intake port 106. A common piston cavity 104C corresponding to the CNG injection valve 120 and the gasoline injection valve 130 is formed in an offset shape on the intake port 106 side in an upper part of a piston 104.