To reduce the number of using potentiometers compared with that in the prior art to reduce a manufacturing cost and to simplify a computing process, in a boom working vehicle having a boom turnable over 360°.

This turning angle detector 73 has a potentiometer 100 for detecting a turning angle of the boom 30 within a continuous angle range less than 360°, a cam 24 and a limit switch 200 as an area detecting means for detecting whether the boom 30 is positioned within a nondetecting area D2 including a nonsensing area D1 of the potentiometer 100 or not, and an outputter 300 for outputting information of the turning angle detected by the potentiometer 100 when the boom 30 is detected to be positioned outside the area D2 by the area detecting means, and for outputting information for indicating that the boom 30 is positioned within the area D2 when the boom 30 is detected to be positioned within the area D2 by the means.