A balance device for an internal combustion engine includes a crankshaft and a balance shaft. The crankshaft includes a CS eccentric weight. The balance shaft includes a BS eccentric weight. A CS connected point deviated from the CS main shaft, and a BS connected point deviated from the BS axial shaft are connected with a connection rod. A CS connection mechanism that enables relative rotation of the crankshaft and the connection rod is provided at the CS connected point. A BS connection mechanism that enables relative rotation of the balance shaft and the connection rod is provided at the BS connected point. A guide section guides a motion of the connection rod so that the balance shaft rotates in an opposite direction to the crankshaft.