A flat plane crankshaft for an in-line four cylinder engine includes eight crank arms. A fourth crank arm and a fifth crank arm are respectively provided with counter weights. Each of a width of the fourth crank arm and a width of the fifth crank arm is configured to be smaller than a width of a second crank arm. Each of a width of the third crank arm and a width of the sixth crank arm is configured to be greater than the width of the second crank arm.