A pulser plate mounting structure is provided in which a key groove (14) is formed in a mounting face (10) of a rotating wall portion (9) formed from a crank arm (3) and a counterweight (4), a key portion (16) for engaging with the key groove (14) is formed on a pulser plate (P) that is superimposed on the mounting face (10), and this pulser plate (P) is secured to the rotating wall portion (9) by means of a securing member (18), wherein the key portion (16) is formed from an arched band-shaped portion (16a) projecting in an arched shape from the pulser plate (P) on one end face side thereof and engaging with the key groove (14), and a pair of connecting portions (16b) for providing integral connection between the pulser plate (P) and opposite ends of the arched band-shaped portion (16a). This provides a high precision of positioning of the pulser plate relative to the crankshaft and a good productivity for the pulser plate.