An engine comprising: a cylinder; a crankcase connected to the cylinder; a crankshaft arranged in the crankcase at least partially, connected via a connecting rod to a piston to make reciprocating motion in the cylinder, and supported by the crankcase so as to be rotatable about a crank axis in conjunction with the reciprocating motion of the piston; a main shaft arranged in the crankcase at least partially, forming a part of a transmission, and supported by the crankcase so as to be rotatable about a main axis in conjunction with the crankshaft; and a balance shaft arranged in the crankcase at least partially, provided on an opposite side of the main shaft across a virtual half line (Lh) starting from the crank axis and extending parallel to an axis (Lc) of the cylinder toward the cylinder as viewed from an axis direction of the crankshaft, and supported by the crankcase so as to be rotatable about a balancer axis in conjunction with the crankshaft, a drive shaft forming a part of the transmission, and supported by the crankcase so as to be rotatable about a drive axis in conjunction with the main shaft, wherein as viewed from the axis direction of the crankshaft, an angle θ1 and an angle θ2 fulfill a relationship of θ1 < θ2, the angle θ1 being provided between the virtual half line (Lh) and a first virtual line segment between the crank axis and the main axis, the angle θ2 is provided between the virtual half line (Lh) and a second virtual line segment between the crank axis and the balancer axis, the angle θ1 is less than 90°, as viewed from the axis direction of the crankshaft, an angle θ3 provided between the virtual half line (Lh) and a third virtual line segment between the crank axis and the drive axis is less than 90°, the angle θ1 and the angle θ3 fulfill a relationship of θ1 < θ3, and the angle θ2 and the angle θ3 fulfill a relationship of θ2 > θ3. a cylinder; a crankcase connected to the cylinder; a crankshaft arranged in the crankcase at least partially, connected via a connecting rod to a piston to make reciprocating motion in the cylinder, and supported by the crankcase so as to be rotatable about a crank axis in conjunction with the reciprocating motion of the piston; a main shaft arranged in the crankcase at least partially, forming a part of a transmission, and supported by the crankcase so as to be rotatable about a main axis in conjunction with the crankshaft; and a balance shaft arranged in the crankcase at least partially, provided on an opposite side of the main shaft across a virtual half line (Lh) starting from the crank axis and extending parallel to an axis (Lc) of the cylinder toward the cylinder as viewed from an axis direction of the crankshaft, and supported by the crankcase so as to be rotatable about a balancer axis in conjunction with the crankshaft, a drive shaft forming a part of the transmission, and supported by the crankcase so as to be rotatable about a drive axis in conjunction with the main shaft, wherein as viewed from the axis direction of the crankshaft, an angle θ1 and an angle θ2 fulfill a relationship of θ1 < θ2, the angle θ1 being provided between the virtual half line (Lh) and a first virtual line segment between the crank axis and the main axis, the angle θ2 is provided between the virtual half line (Lh) and a second virtual line segment between the crank axis and the balancer axis, the angle θ1 is less than 90°, as viewed from the axis direction of the crankshaft, an angle θ3 provided between the virtual half line (Lh) and a third virtual line segment between the crank axis and the drive axis is less than 90°, the angle θ1 and the angle θ3 fulfill a relationship of θ1 < θ3, and the angle θ2 and the angle θ3 fulfill a relationship of θ2 > θ3.