A fixing device for structure members, capable of preventing first and second engagement members from coming off from engagement grooves of a pair of structure members. First and second engagement members (4, 4') are held by a holding member (5) so as to be movable in the width direction of an engagement groove (A1). Between the first and second engagement members (4, 4') is provided a coil spring (7) urging the engagement members in the direction to separate the members from each other. The coil spring (7) causes the first engagement member (4) to butt against a projection section (A2) of the engagement groove (A1) of one structure member (A) and against a projection section of an engagement groove of the other structure member (not shown), and also causes the second engagement member (4') to butt against a projection section (A3) of the engagement groove (A1) of the structure member (A) and against a projection section of an engagement groove of the other structure member. In this state, each of the engagement ridges (4e, 4e) of the first and second engagement members (4, 4') individually faces the projection sections (A2, A3) of the one engagement groove (A1) and the projection sections of the other engagement groove, and the first and second engagement members (4, 4') are unable to come off from the one engagement groove (A1) and the other engagement groove.