How to get the blanck available value in IDENTITY column

Try this:

DECLARE @tmp TABLE (ID INT)

INSERT INTO @tmp (ID)
VALUES(1)
INSERT INTO @tmp (ID)
VALUES(2)
INSERT INTO @tmp (ID)
VALUES(3)
INSERT INTO @tmp (ID)
VALUES(4)
INSERT INTO @tmp (ID)
VALUES(15)
INSERT INTO @tmp (ID)
VALUES(16)
INSERT INTO @tmp (ID)
VALUES(17)

DECLARE @mId INT

SELECT @mId = MAX(ID)
FROM @tmp

;WITH NewIds AS
(
	SELECT 1 AS nID 
	UNION ALL
	SELECT nID + 1 
	FROM NewIds
	WHERE nID<@mId
)
SELECT ni.nID
FROM NewIds AS ni 
WHERE ni.nID NOT IN (SELECT ID AS nID FROM @tmp)

In above code CTE[^] were used.

result:

nID
5
6
7
8
9
10
11
12
13
14

[EDIT]
By The Way... Bad table design! Please, alter your table, but first read it: IDENITY[^]

Example table design:

CREATE TABLE MyTable (ID INT INDENTITY(1,1), Column2 VARCHAR(30), Column3 INT)

[/EDIT]