A crankshaft mechanism for an engine wherein a crankshaft can be disposed close to the piston side while securing a moment of inertia. In the crankshaft mechanism for the engine including the piston, the crankshaft is provided with a crank pin and counterweight parts, and a connecting rod connecting the piston and the crank pin of the crankshaft to each other, the inner side of a circumferential end, on the opposite side of the crank pin, of each of the counterweight parts is cut out along a curve at equidistance R from the center Q of the crank pin, to obtain such a shape so as to avoid a projected part, projected in the direction of the piston, at the lower end of a small end part of the connecting rod when the piston reaches the bottom dead center.