To provide a transmitter of a tire condition monitoring device having a valve stem that can be securely and stably attached to a main body.

The transmitter includes a main body 10 positioned inside a tire, and a valve stem 11 extending from the main body 10. A resin case 13 of the main body 10 has a cylindrical body 20. A metallic cylindrical collar 21 is inserted into the cylindrical body 20. With the inserted part 22 of the valve stem 11 inserted in the collar 21, a nut 24 is threadedly attached to the internal thread 22a of the inserted part 22 and fastened. As a result, the cylindrical body 20 and the collar 21 are clamped by the stepped portion 11a of the valve stem 11 and the nut 24. The force to fasten the nut 24 is received mainly by the collar 21 and scarcely acts on the cylindrical body 20.