An oscillating gear device includes an axially fixed gear piece rotatable around a first axis and an axially oscillating gear piece that oscillates while being inclined with respect to the first axis. A plurality of pins held by one of the gear pieces engage with a plurality of tooth grooves of the other gear piece in engagement regions at a predetermined length in longitudinal directions of the pins. A formula rC0/rF0=ZC/ZF is satisfied, where rC0 is a distance between an arbitrary point on the central axis of the pin in a corresponding section corresponding to the engagement regions of the pin as viewed in the radial direction of the pin and the first axis, rF0 is a distance between the arbitrary point and the second axis, ZC is the number of pins of the one gear piece, and ZF is the number of tooth grooves of the other gear piece.