Ball screw device, linear actuator and vehicle steering system

A ball screw device comprising:

• a ball nut that includes a nut body having a female thread groove;
• a male shaft that has a male thread groove facing the female thread groove;
• a spiral raceway that is defined between part of the female thread groove and part of the male thread groove;
• a plurality of balls that are arranged in the raceway;
• and
• a bridge that is retained in an insertion hole formed in the ball nut and that is used to circulate the balls from one end of the raceway to the other end of the raceway, wherein
• a plurality of the raceways and a plurality of the bridges are provided in an axial direction of the male shaft,
• the ball nut includes an annular flange that is formed on an outer periphery of the nut body,
• the annular flange has a plurality of mounting portions in a circumferential direction of the annular flange, a predetermined fixing member being fitted to each mounting portion,
• the plurality of bridges include at least two first bridges that are arranged so as to overlap with the annular flange in position in the axial direction, and
• the first bridges each are arranged at a position different in the circumferential direction from positions at which the mounting portions are arranged, and are arranged at unequal intervals in the circumferential direction.