The seat fell vacant after M Venkaiah Naidus election as the country's vice president on August 5.
Secretary of the legislative assembly Prithvi Raj said Kannanthanamwas elected unopposed as no other nomination was filed for the seat.
State parliamentary secretary Rajendra Rathore, social justice and empowerment minister Arun Chaturvedi, the BJPs state president Ashok Parnami and others were present during the occasion.
Alphons, a 1979 batch IAS officer, began his political journey by becoming an independent MLA backed by the CPI(M) in Kerala in 2006 but joined the BJP in 2011.
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