What's wrong with my Multi-Thread Echo Server ?

I've just coded a simple Multi-thread Echo Server (with POSIX pthread)

First client is served well, but from second client cannot be served at all.
all clients connect from 127.0.0.1 (localhost).

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <pthread.h>

void echo(int sock);

int
main(int argc,char** argv){
int sock; // socket
int ac_sock; // socket which is accepted.
struct sockaddr_in soaddr_server; // for server
struct sockaddr_in soaddr_client; // for client
struct hostent *hp; // for client
int len_soaddr_client;
int z;
int k;
pthread_t t_1;

sock = socket(PF_INET,SOCK_STREAM,0);
if( sock == -1 )
printf("error socket()\n");

memset(&soaddr_server,0,sizeof soaddr_server);

soaddr_server.sin_family = AF_INET;
soaddr_server.sin_port = htons(9005);
inet_aton("127.0.0.1",(struct in_addr *)&soaddr_server.sin_addr);
if ( soaddr_server.sin_addr.s_addr == INADDR_NONE )
printf("error sin_addr\n");

z = bind(sock,(struct sockaddr *)&soaddr_server,sizeof soaddr_server);
if( z == -1 )
printf("error bind() : %s\n",strerror(errno));

for(;;){

z = listen(sock,5);

len_soaddr_client = sizeof soaddr_client;
ac_sock = accept(sock,(struct sockaddr *)&soaddr_client,&len_soaddr_client);
hp = gethostbyaddr((char*)&soaddr_client.sin_addr,
sizeof soaddr_client.sin_addr,
soaddr_client.sin_family);

printf("Connected by %s\n",(char *)hp->h_name);

pthread_create(&t_1,NULL,(void *)echo,(void *)ac_sock);
pthread_join(t_1,NULL);

}

return 0;
}

void echo(int ac_sock){
int k;
char buf[255];

while( (k = read(ac_sock,buf,sizeof buf)) > 0 ){
printf("%s",buf);
fflush(stdout);
write(ac_sock,buf,k);
}

close(ac_sock);

}