The present invention relates to a lower leg rest adjusting device and an infant carrier therewith for adjusting an angle of a lower leg rest pivoted to a body of an infant carrier. The leg rest adjusting device includes an operating component, a pair of angle-fixing structures and a pair of linking components. Two ends of the lower leg rest adjusting device are pivoted to the body by the pair of angle-fixing structures, and the pair of angle-fixing structures selectively being engaged with the lower leg rest. An end of each linking component is connected to the operating component, the other end of each linking component is connected to the corresponding angle-fixing structure, the operating component driving the pair of linking components to pull the pair of angle-fixing structures to be unlocked from the lower leg rest so as to adjust the angle of the lower leg rest.