The present invention aims to provide a combustion chamber structure for an engine, in which a configuration of a cavity of a piston, etc., are suitably designed to surely start combustion of fuel at a predetermined ignition timing after the fuel is injected at a predetermined fuel injection start timing. The combustion chamber structure for an engine, wherein the structure includes a piston (10) formed with a cavity (11), a fuel injector (3) arranged on the central axis of the piston (10) for injecting fuel toward the cavity (11), and ignition plugs (4) provided above the cavity (11). A radius of the cavity, a depth of the cavity, and each of positions of the ignition plugs (4) are designed so that a spray conveying distance by which a mixture gas containing the fuel travels from a fuel injection start timing of the fuel injector (3) to an ignition timing of the ignition plug (4) becomes equal to or longer than a length of a spray conveying path through which the fuel injected from the fuel injector (3) reaches each ignition plug (4) via the cavity (11).