To provide an assistant light source and a front-light emitting light to the light guide plate efficiently. A sectional shape of prism 14a in the center of light stick is isosceles triangle having a tip angle (T.degree.) of 100.degree. and a tilt angle (a.sub.1) of 40.degree.. The depth of the prism 14a is D.sub.1 .mu.m. A sectional shape of prism 14b in a 1/4 position from an end of light stick is triangle having a tip angle (T.degree.) of 100.degree. and a tilt angle (a.sub.2) of 34.8.degree.. The depth of the prism 14b is D.sub.2 .mu.m. A sectional shape of prism 14c in an end of light stick is triangle having a tip angle (T.degree.) of 100.degree. and a tilt angle (a.sub.3) of 24.05.degree.. The depth of the prism 14c is D.sub.3 .mu.m.