An interlocking device for locking two modular building formworks, each of the modular building formworks including a wall plate and a frame disposed on a mounting surface of the wall plate, the frame having an aligning hole and a locking hole, said interlocking device comprising: a housing unit that can be securely disposed on one of the frames; a rotation transferring unit disposed within said housing unit and having an input shaft which extends along and is rotatable about a first axis, and an output shaft which extends along a second axis that is perpendicular to the first axis, said output shaft being rotatable about the second axis with the rotation of said input shaft; a transmitting unit coupled with and movable relative to said output shaft along the second axis and being rotatable about the second axis with the rotation of said output shaft; and a locking member coupled with said transmitting unit and movable along and rotatable about the second axis with the movement and rotation of said transmitting unit, said locking member having a head portion which is coupled with said transmitting unit along the second axis, a positioning portion which extends from said head portion along the second axis and away from said transmitting unit, and a threaded portion which extends from said positioning portion along the second axis and away from said head portion, wherein said locking member is insertable into the aligning hole of one of the frames to have said head portion abutting against the one frame, and said threaded portion is matingly engageable in the locking hole of the other one of the frames so as to lock the two modular building formworks, characterized in that: said positioning portion of said locking member is tapered gradually from said head portion along the second axis and has a maximum outer diameter which is smaller than an outer diameter of said head portion, said positioning portion having a first tapered section which is connected with said head portion, and a second tapered section which is connected between said first tapered section and said threaded portion, wherein said first tapered section is engageable T in the aligning hole of one of the frames and said second tapered section is engageable in the locking hole of the other one of the frames when said locking member is inserted into the aligning hole of the one frame and said threaded portion is threadedly engaged in the locking hole of the other frame. a housing unit that can be securely disposed on one of the frames; a rotation transferring unit disposed within said housing unit and having an input shaft which extends along and is rotatable about a first axis, and an output shaft which extends along a second axis that is perpendicular to the first axis, said output shaft being rotatable about the second axis with the rotation of said input shaft; a transmitting unit coupled with and movable relative to said output shaft along the second axis and being rotatable about the second axis with the rotation of said output shaft; and a locking member coupled with said transmitting unit and movable along and rotatable about the second axis with the movement and rotation of said transmitting unit, said locking member having a head portion which is coupled with said transmitting unit along the second axis, a positioning portion which extends from said head portion along the second axis and away from said transmitting unit, and a threaded portion which extends from said positioning portion along the second axis and away from said head portion, wherein said locking member is insertable into the aligning hole of one of the frames to have said head portion abutting against the one frame, and said threaded portion is matingly engageable in the locking hole of the other one of the frames so as to lock the two modular building formworks, characterized in that: said positioning portion of said locking member is tapered gradually from said head portion along the second axis and has a maximum outer diameter which is smaller than an outer diameter of said head portion, said positioning portion having a first tapered section which is connected with said head portion, and a second tapered section which is connected between said first tapered section and said threaded portion, wherein said first tapered section is engageable T in the aligning hole of one of the frames and said second tapered section is engageable in the locking hole of the other one of the frames when said locking member is inserted into the aligning hole of the one frame and said threaded portion is threadedly engaged in the locking hole of the other frame. said positioning portion of said locking member is tapered gradually from said head portion along the second axis and has a maximum outer diameter which is smaller than an outer diameter of said head portion, said positioning portion having a first tapered section which is connected with said head portion, and a second tapered section which is connected between said first tapered section and said threaded portion, wherein said first tapered section is engageable T in the aligning hole of one of the frames and said second tapered section is engageable in the locking hole of the other one of the frames when said locking member is inserted into the aligning hole of the one frame and said threaded portion is threadedly engaged in the locking hole of the other frame.