To obtain optimum light quantity by absorbing positional deviation at both end parts of an illumination lamp, to prevent the supporting member of the holding means of the illumination lamp from being distorted and to prevent the positional deviation of the illumination lamp from occurring even when impact is imposed.

One end part of the fluorescent lamp 21 is positioned and fixed and the other end part is locked to a lamp holder 41 having electrode armatures 43a and 43b. The holder 41 is engaged with a base 44 and it can be rotated within a prescribed range. Then, the deviation of the other end part of the lamp 21 in the circumferential direction with respect to one positioned end part is absorbed by the rotation of the holder 41 with respect to the base 44.