A linear motion mechanism comprises a second rotating shaft having an external thread portion 48a at least at a part of an area in a length direction and an adjusting pin 40 having an opening 60 through which the external thread portion 48a passes. The opening 60 of the adjusting pin 40 has a cross-sectional shape that enables the adjusting pin 40 to move in a radial direction of the second rotating shaft. An inner thread part 62 that engages with and disengages from the external thread portion 48a of the second rotating shaft depending on a position of the adjusting pin 40 is formed on an inner surface of the opening 60. A spring 56 capable of retaining a state in which the external thread portion 48a and the inner thread part 62 engage with each other is arranged on the adjusting pin 40.