The Representation of the People Act,1951
Section 149. Casual vacancies in the House of the people
(1) When the seat of a member elected to the House of the People becomes vacant or is declared vacant or his election to the House of the People is declared void, the Election Commission shall, subject to the provisions of sub-section (2) by a notification in the Gazette of India, call upon the Parliamentary constituency concerned to elect a person for the purpose of filling the vacancy so caused before such date as may be specified in the notifications and the provisions of this Act and of the rules and orders made thereunder shall apply, as for as may be, in rotation to the election of a member to fill such vacancy.
(2) If the vacancy so caused be a vacancy in a seat reserved in any such constituency for the Scheduled Castes or for any Scheduled Tribes, the notification issued under subsection (1) shall specify that the person to fill that seat shall belong to the Scheduled Castes or to such Scheduled Tribes, as the case may be.