A start assister (10) of a fuel injection pump, comprising a piston (46) inlayed into a housing (H), an upper chamber (49) formed over the piston (46), lower chamber (48) formed under the piston (46), wherein a sub port (42) is opened and closed by sliding of the piston (46), and a fuel injection timing is advanced by closing the sub port (42). A thermo-element (61) for sliding the piston is arranged on one side of the piston (46) in sliding direction. A telescopic pin (61a) of the thermo-element (61) is arranged in a low-pressure chamber (50) divided into the upper chamber and the lower chamber, and a connection pin (62) is interposed between the telescopic pin (61a) and the piston (46).