To provide a diaphragm device for a projector moving two diaphragm plates in opposite direction respectively and stopping them at positions some interval apart.

A first gear unit 2 includes a partial circular arc portion 22 and a partially-toothless gear portion 21. A second gear unit 1 includes a circular arc portion 12 having a plurality of recessed circular arcs in the circumferential direction corresponding to the partial circular arc portion 22 of the first gear unit 2, and a gear 11 and a pinion 13 corresponding to the partially-toothless gear portion 21 of the first gear unit 2. When the partial circular arc portion 22 of the first gear unit 2 abuts on the circular arc portion 12, even if the first gear unit 2 rotates, the gear 11 does not rotate. When the abutment is released, the teeth of the partially-toothless gear portion 21 of the first gear unit 2 are threadedly engaged with the gear 11 to transmit rotation of the first gear unit 2 to the gear 11. Two diaphragm plates 6 and 7 each on which a rack 64 or 65 threadedly engaged with the pinion 13 of the second gear unit 1 is formed, move in directions opposed to each other according to rotation of the pinion 13 of the second gear unit 1.