Provided are a wide angle optical system having better optical performance, low in cost, and compact in size, an imaging lens device having the wide angle optical system, a monitor camera, and a digital apparatus. The wide angle optical system (1) has, in order from the object side to the image side, a first lens (11) having a negative optical power, a second lens (12) having a negative optical power, a third lens (13) having a positive optical power, an aperture (15), and a fourth lens (14) having a positive optical power. The wide angle optical system satisfies the conditional expression of 3