A micrometer, comprising: a main body of a substantially U-shape that has an anvil at a first end thereof and a female screw at a second end thereof; a spindle that is screwed in the female screw and is configured to be axially moved toward and away from the anvil in accordance with a screwing rotation; an operation section that is configured to rotate the spindle to move the spindle toward and away from the anvil; and a constant pressure mechanism interposed between the operation section and the spindle, the constant pressure mechanism being configured to transmit a rotation of the operation section to the spindle and to freely rotate the operation section relative to the spindle when more than a predetermined load is applied on the spindle, wherein the operation section includes a ratchet ring of a ring shape that is provided with a sawtooth first projection on an inner circumference thereof; the constant pressure mechanism includes: a support shaft provided with a shaft portion that is provided at a base end of the spindle coaxially with a center axis of the spindle and inserted into the ratchet ring and a head provided at a base end of the shaft portion and having a diameter larger than the shaft portion; a further shaft held by the head; a ratchet claw that is provided in the ratchet ring in a manner to be rotatable around said shaft held by the head and is provided with a second projection to be engaged with the first projection on a lateral side thereof; and a biasing unit that is configured to bias the ratchet claw toward the first projection. a main body of a substantially U-shape that has an anvil at a first end thereof and a female screw at a second end thereof; a spindle that is screwed in the female screw and is configured to be axially moved toward and away from the anvil in accordance with a screwing rotation; an operation section that is configured to rotate the spindle to move the spindle toward and away from the anvil; and a constant pressure mechanism interposed between the operation section and the spindle, the constant pressure mechanism being configured to transmit a rotation of the operation section to the spindle and to freely rotate the operation section relative to the spindle when more than a predetermined load is applied on the spindle, wherein the operation section includes a ratchet ring of a ring shape that is provided with a sawtooth first projection on an inner circumference thereof; the constant pressure mechanism includes: a support shaft provided with a shaft portion that is provided at a base end of the spindle coaxially with a center axis of the spindle and inserted into the ratchet ring and a head provided at a base end of the shaft portion and having a diameter larger than the shaft portion; a further shaft held by the head; a ratchet claw that is provided in the ratchet ring in a manner to be rotatable around said shaft held by the head and is provided with a second projection to be engaged with the first projection on a lateral side thereof; and a biasing unit that is configured to bias the ratchet claw toward the first projection. a support shaft provided with a shaft portion that is provided at a base end of the spindle coaxially with a center axis of the spindle and inserted into the ratchet ring and a head provided at a base end of the shaft portion and having a diameter larger than the shaft portion; a further shaft held by the head; a ratchet claw that is provided in the ratchet ring in a manner to be rotatable around said shaft held by the head and is provided with a second projection to be engaged with the first projection on a lateral side thereof; and a biasing unit that is configured to bias the ratchet claw toward the first projection.