An imaging optical system comprising a first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) having a negative refractive power, a second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002) having a positive refractive power, an aperture stop (104, 204, 304, 404, 504, 604, 704, 804, 904, 1004) and a third lens (103, 203, 303, 403, 503, 603, 703, 803, 903, 1003) having a positive refractive power, which are arranged from the object side to the image side, wherein the imaging optical system comprises no more than three lenses, characterized in that when distance between the image-side principal point of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) and the object-side principal point of the second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002) is represented by |d|, signed distance is represented by d = -|d| when the image-side principal point of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) is located closer to the image than the object-side principal point of the second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002), and the composite focal length of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) and the second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002) is represented by f12, the relationships d < 0 and 0.005 < d / f 12 < 16 are established, and wherein when the common principal axis of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001), the second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002) and the third lens (103, 203, 303, 403, 503, 603, 703, 803, 903, 1003) is defined as the optical axis, distance between the point on the optical axis and on the image-side surface of the third lens (103, 203, 303, 403, 503, 603, 703, 803, 903, 1003) and the image plane is represented by t3, and distance between the point on the optical axis and on the object-side surface of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) and the point on the optical axis and on the image-side surface of the third lens (103, 203, 303, 403, 503, 603, 703, 803, 903, 1003) is represented by t, the relationship t 3 / t > 0.5 is established. wherein the imaging optical system comprises no more than three lenses, characterized in that when distance between the image-side principal point of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) and the object-side principal point of the second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002) is represented by |d|, signed distance is represented by d = -|d| when the image-side principal point of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) is located closer to the image than the object-side principal point of the second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002), and the composite focal length of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) and the second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002) is represented by f12, the relationships d < 0 and 0.005 < d / f 12 < 16 are established, and wherein when the common principal axis of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001), the second lens (102, 202, 302, 402, 502, 602, 702, 802, 902, 1002) and the third lens (103, 203, 303, 403, 503, 603, 703, 803, 903, 1003) is defined as the optical axis, distance between the point on the optical axis and on the image-side surface of the third lens (103, 203, 303, 403, 503, 603, 703, 803, 903, 1003) and the image plane is represented by t3, and distance between the point on the optical axis and on the object-side surface of the first lens (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001) and the point on the optical axis and on the image-side surface of the third lens (103, 203, 303, 403, 503, 603, 703, 803, 903, 1003) is represented by t, the relationship t 3 / t > 0.5 is established.