A balancer mechanism applied to a straight engine, the engine including: an odd number of three or more cylinders; a crankshaft having a crank journal; crank arms arranged in the crank journal in correspondence with the respective cylinders and spaced apart at a predetermined angular interval; a reciprocating portion and a rotating portion each having a mass such that an inertia couple is generated in the crankshaft; and a crank weight (21, 22, 23) arranged in the crankshaft in order to reduce some of the inertia couple, thereby producing a combined inertia couple in the crankshaft, the balancer mechanism comprising: a balance shaft extending parallel to the crank journal and rotating in a direction opposite to a rotational direction of the crank journal at a speed equal to a rotational speed of the crank journal; and a balance weight (25, 26) arranged in the balance shaft in order to cancel the combined inertia couple, characterized in that the axes of the cylinders are arranged to be at an offset (d) with respect to the axis of the crank journal, and the phase of a rotation angle of the balance shaft with respect to a rotation angle of the crank journal is retarded by a predetermined angle (+α) with respect to such a reference phase that can cancel the combined inertia force generated in a case where the axes of the cylinders cross the axis of the crank journal. an odd number of three or more cylinders; a crankshaft having a crank journal; crank arms arranged in the crank journal in correspondence with the respective cylinders and spaced apart at a predetermined angular interval; a reciprocating portion and a rotating portion each having a mass such that an inertia couple is generated in the crankshaft; and a crank weight (21, 22, 23) arranged in the crankshaft in order to reduce some of the inertia couple, thereby producing a combined inertia couple in the crankshaft, the balancer mechanism comprising: a balance shaft extending parallel to the crank journal and rotating in a direction opposite to a rotational direction of the crank journal at a speed equal to a rotational speed of the crank journal; and a balance weight (25, 26) arranged in the balance shaft in order to cancel the combined inertia couple, characterized in that the axes of the cylinders are arranged to be at an offset (d) with respect to the axis of the crank journal, and the phase of a rotation angle of the balance shaft with respect to a rotation angle of the crank journal is retarded by a predetermined angle (+α) with respect to such a reference phase that can cancel the combined inertia force generated in a case where the axes of the cylinders cross the axis of the crank journal. a balance shaft extending parallel to the crank journal and rotating in a direction opposite to a rotational direction of the crank journal at a speed equal to a rotational speed of the crank journal; and a balance weight (25, 26) arranged in the balance shaft in order to cancel the combined inertia couple, characterized in that the axes of the cylinders are arranged to be at an offset (d) with respect to the axis of the crank journal, and the phase of a rotation angle of the balance shaft with respect to a rotation angle of the crank journal is retarded by a predetermined angle (+α) with respect to such a reference phase that can cancel the combined inertia force generated in a case where the axes of the cylinders cross the axis of the crank journal.